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GENETICS  AND  EUGENICS 

A  TEXT-BOOK  FOR  STUDENTS  OF  BIOLOGY  AND 

A  REFERENCE  BOOK  FOR  ANIMAL 

AND  PLANT  BREEDERS 


BY 

W. E.  CASTLE 

PROFESSOR  OF  ZOOLOGY  IN  HARVARD  UNIVERSITY  AND 

RESEARCH  ASSOCIATE  OF  THE  CARNEGIE 

INSTITUTION  OF  WASHINGTON 


Private  Prot:jerfy  of 

^^  P.  METCALF 


»yjaaaiait«»i*r*tittfcM 


/ 


CAMBRIDGE 
HARVARD  UNIVERSITY  PRESS 

LONDON:  HUMPHREY  MILFORD 

Oxford  University  Press 

1921 


COPYRIGHT,   1916 
HARV.\ED   UNIVERSITY   PRESS 

First  impression  issued  December,  1916 

Second  impression  issued  February,  1917 

Third  impression  issued  July,  1917 

Second  Edition 

First  impression  issued  August,  1920 
Second  impression  issued  August,  1921 


PREFACE 

This  book  is  an  attempt  to  present,  in  a  form  as  simple  and 
readily  intelligible  as  possible,  the  subject  of  heredity,  as 
related  to  man  and  his  creatures,  the  domestic  animals  and 
cultivated  plants.  To  write  such  a  book  has  been  with  the 
author  a  long  cherished  ambition,  but  one  which,  as  the  years 
went  by,  seemed  less  and  less  likely  of  realization,  as  knowl- 
edge of  the  subject  increased  and  took  on  more  and  more 
complicated  forms.  Each  year,  however,  he  has  been 
forced  by  his  responsibilities  as  a  teacher,  to  make,  for 
students  having  only  an  elementary  knowledge  of  biology, 
an  analysis  and  summary  of  our  knowledge  of  this  subject 
to  date.  The  longer  he  has  continued  to  do  this,  the  more 
fully  he  has  realized  that  a  subject  in  a  state  of  healthy 
growth  can  never  assume  a  final  and  finished  form.  He 
makes  no  apology,  therefore,  for  presenting  the  subject 
with  very  unevenly  and  incompletely  developed  parts. 
Such,  it  must  be  confessed,  is  the  present  state  of  our  knowl- 
edge. 

It  would  be  a  great  service  to  the  student  to  show  him 
where  in  his  subject  positive  knowledge  stops  and  specula- 
tion, the  useful  servant  but  dangerous  master  in  science, 
begins.  This  task,  where  possible,  has  been  attempted  in 
this  book.  But  such  attempts  can  of  necessity  succeed  only 
partially  and  for  the  time  being,  for  it  often  happens  that 
the  speculation  of  today  becomes  the  verified  theory  of 
tomorrow.  For  having  guessed  right  and  proved  the  cor- 
rectness of  their  guesses,  we  honor  in  this  field  the  names  of 
Lamarck,  Darwin,  Weismann,  and  Mendel.  Others  still 
living  have  made  contributions  of  scarcely  less  importance 
but  to  name  them  would  be  invidious.  Americans  may  take 
encouragement  from  the  thought  that  all  are  not  likely  to 
be  named  from  one  side  of  the  Atlantic  and  later  enumera- 


W-^ 


V 

19653 


iv  PREFACE 

tions  are  likely  to  include  names  from  Pacific  lands  also. 
For  advance  in  science  never  results  merely  from  brilliant 
guesses  by  the  few,  but  takes  place  chiefly  through  the 
patient,  persistent  efforts  of  numerous  workers  who  test  by 
observation  and  experiment  every  suggested  explanation  of 
the  phenomena  of  nature.  This  is  a  task  of  such  magnitude 
and  such  importance  that  in  it  the  cooperation  of  all  nations 
is  needed  and  fortunately  is  not  withheld.  To  promote  the 
common  good  of  all  is  the  greatest  honor  of  each. 

The  author  has  found  that  interest  in  the  subject  of 
heredity  is  not  confined  to  college  classes  but  is  shared  by 
people  of  intelligence  everywhere,  because  it  touches  and 
affects  the  lives  of  all.  The  animal  breeder  and  the  plant 
breeder  have  an  intensified  interest  in  the  subject  because  it 
vitally  concerns  the  success  or  failure  of  their  occupations. 
The  needs  of  this  wider  public  have  been  kept  in  mind  in  the 
preparation  of  this  book,  but  it  has  not  been  thought  neces- 
sary to  omit  on  this  account  discussion  of  questions  re- 
quiring thoughtful  consideration  for  their  full  understanding. 
A  discussion  which  evokes  no  independent  thinking,  or  even 
opposition,  is  not  likely  to  extend  knowledge,  the  teacher's 
prime  concern. 

1  am  indebted  to  many  friends  and  fellow  biologists  for 
assistance  in  connection  with  the  illustrations,  acknowl- 
edged in  the  legends  of  the  figures,  to  Professor  B.  M.  Davis 
for  a  critical  revision  of  Chapter  VI,  and  to  Professor  J.  A. 
Detlefsen  for  assistance  in  revising  the  proofs.  My  best 
thanks  are  due  to  the  publishers  who  have  spared  no  effort 
to  make  their  part  of  the  work  successful. 

W.  E.  Castle. 

Cambridge,  Massachusetts, 
December,  1916. 


PREFACE  TO  SECOND  EDITION 

Rapid  advance  in  our  knowledge  of  the  fundamental  prin- 
ciples of  genetics  has  made  necessary  a  complete  rewriting  of 
several  chapters  as  originally  published  and  the  addition  of 
several  others.  The  more  important  changes  and  additions 
relate  to  the  subjects  of  blending  inheritance,  the  pure  line 
principle,  the  nature  of  genetic  changes,  their  frequency  and 
location  in  the  germ-cells,  linkage,  inbreeding,  and  heterosis. 

W.  E.  C. 

March,  1920. 


^ 


CONTENTS 


INTRODUCTION 


PAGE 

3 


PART  I.   GENETICS 

I.    Darwin's  Theory  of  Evolution  and  its  Evidences  7 
11.    Contributions  of  Lamarck,  Weismann,  and  Herbert 
Spencer  to  the  Theory  of  Evolution;  Darwin's 

Theory  of  Pangenesis 18 

III.  Are  Acquired  Characters  Inherited  .^ 28 

IV.  Weismann's  Theory  of  Heredity 47 

V.    Attempts  to  Classify  and  Measure  Variation  :  Biom- 
etry    b^ 

VI.    The  Mutation  Theory 71 

VII.    The  Pioneer  Plant  Hybridizers  :  the  Discovery  and 

Rediscovery  of  Mendel's  Law 82 

VIII.    Mendel's  Law  of  Heredity  Illustrated  in  Animal 

Breeding 88 

IX.    Some  Mendelian  Terms  and  their  Uses 98 

X.    Calculating  Mendelian  Expectations 104 

XI.    Modified  Mendelian  Ratios;  Heterozygous  Char- 
acters; Atavism  or  Reversion 109 

XII.    The  Unit-Characters  of  Rodents l^^a 

XIII.  Unit-Characters  in  Cattle  and  Horses 130 

XIV.  Unit-Characters  in  Swine,  Sheep,  Dogs  and  Cats  137 
XVv    Unit-Characters  in  Poultry  and  in  Plants    .    .    .  145 

XVI.    Unit-Characters  of  Insects 154 

XVII.    Sex-Linked  and  other  Kinds  of  Linked  Inheritance 

in  Drosophila 159 

XVIII.    Drosophila  Type  and  Poultry  Type  of  Sex-Linked 

Inheritance 164 

XIX.    Linkage , 107 

XX.    The  Nature  of  Genes      177 

XXI.    Are  Unit-Characters  (Genes)  Constant  or  Variable?  182 
XXII.    Inheritance  of  Size  and  other  Quantitative  Char- 
acters.    The  Hypothesis  of  Multiple  Factors  190 

XXIII.  Genetic  Changes  and  the  Chromosomes 205 

XXIV.  Genetic  Changes  in  Asexual  Reproduction,  in  Par- 

thenogenesis, AND  IN  Self-Fertilization      .    .    .  209 

vii 


Till  CONTEXTS 

XXV.    Genetic  Changes  in  Bisexual  Reproduction  ...  219 

XXVI.    Inbreeding  and  Cross-Breeding 2-27 

XXVII.    Hybrid  A'igor  or  Heterosis 242 

XXVIII.     Galton's  Law  of  Ancestral  Heredity  and  his  Prin- 
ciple of  Regression 246 

XXIX.    Sex  Determination 248 

TART  II.    EUGENICS 

XXX.    Human  Crosses 20.5 

XXXI.    Physical  and  Mental  Inheritance  in  Man      ...  271 
XXXII.    Heredity  of  General  Mental  Ability',  Insanity. 

Epilepsy',  and  Feeble-Mindedness 279 

XXXIII.    The  Possibility'  and  Prospects  of  Breeding  a  Better 

Human  Race C92 

APPENDIX.   Translation  of  Mendel's  Paper,  Experiments 

IN  Pl.\nt-Hybridisation 313 

BIBLIOGRAPHY 355 

INDEX 391 


GENETICS  AND  EUGENICS 


t 


\ 


INTRODUCTION 

Genetics  may  be  defined  as  the  science  which  deals  with  the 
coming  into  being  of  organisms.  It  does  not  refer,  however, 
to  the  first  creation  of  organic  beings,  but  rather  to  the  pres- 
ent and  e very-day  creation  of  new  individuals  or  new  races. 
It  refers  particularly  to  the  part  that  parent  organisms  have 
in  bringing  new  organisms  into  being  and  to  the  influence 
which  parents  exert  on  the  characteristics  of  their  offspring. 
In  this  sense  it  is  nearly  equivalent  to  the  term  heredity. 
But  logically,  though  less  immediately,  it  is  concerned  with 
all  agencies  which  in  any  way  affect,  condition,  or  limit  the 
coming  into  being  of  a  new  organism  or  a  new  race.  All 
physical  and  chemical  changes  in  the  world  outside  the  organ- 
ism, or  in  a  word  the  environment,  vitally  concern  genetics, 
though  they  are  the  more  immediate  field  of  study  of  other 
branches  of  biology. 

Eugenics,  from  its  et;yTiiology,  means  coming  into  being  well. 
It  is  used  at  present  solely  with  reference  to  man,  and  means 
almost  literally  the  science  of  being  well-born.  Since  man  is 
zoologically  merely  one  of  the  higher  animals,  it  is  evident 
that  his  reproduction  is  a  very  special  case  falling  under  the 
general  laws  of  genetics,  and  before  we  can  properly  under- 
stand this  special  case  we  must  know  something  of  the  general 
laws  of  genetics.  We  shall  therefore  turn  our  attention  to 
genetics  first  and  foremost,  and  to  eugenics  subsequently  and 
secondarily. 

The  term  Eugenics  was  proposed  by  Francis  Galton  who 
defines  it  thus:  —  "  Eugenics  is  the  study  of  agencies  under 
social  control  that  may  improve  or  impair  the  racial  qualities 
of  future  generations,  either  physically  or  mentally." 

As  thus  defined  it  is  purely  an  applied  science,  for  it  is  con- 
cerned only  with  those  agencies  which  are  under  social  con- 
trol and  gives  no  attention  to  any  agencies,  however  impor- 


4    '  GENETICS  AND  EUGENICS 

tant,  which  are  not  under  social  control.  Its  scope  therefore 
is  much  narrower  than  that  of  genetics.  It  is  concerned  with 
only  so  much  of  genetics  as  concerns  man,  and  with  only  so 
much  of  that  as  is  under  social  control.  To  determine  what 
are  the  general  principles  of  genetics  and  to  what  extent  man 
is  subject  to  them  are  primarily  biological  problems,  but  to 
determine  how  far  these  are  socially  controllable  is  a  problem 
for  the  sociologist,  and  one  which  I  shall  not  attempt  to 
answer  without  help  from  sociologists. 

The  coming  into  being  of  a  new  organism  is  one  of  the 
least  understood  of  all  natural  phenomena.  Even  to  the 
trained  biologist  it  is  largely  an  unexplained  mystery.  To 
understand  his  vie^'point  concerning  it,  and  what  definite 
facts  he  knows  about  it,  and  how  he  attempts  to  explain 
them,  we  must  be  familiar  with  certain  of  the  generalizations 
of  biology.  Familiarity  with  the  more  important  of  these 
fundamental  generalizations  of  biology  will  be  assumed  in  the 
present  work. 

From  the  philosophical  standpoint  genetics  is  only  a  sub- 
division of  evolution.  For  the  evolution  theory  teaches  that 
the  organisms  now  existing  have  come  into  being  through 
descent  with  modification  from  those  which  existed  at  an 
earlier  time  and,  in  general,  that  the  world  as  we  know  it  today 
is  different  from  what  it  has  been  at  any  previous  time;  that 
all  things,  organic  and  inorganic,  are  constantly  undergoing 
change,  yet  nothing  wholly  new  comes  into  being,  for  every- 
thing new  arises  out  of  something  which  existed  before.  Thus 
no  new  matter  is  created,  yet  new  creations  constantly  arise 
out  of  elements  which  before  existed  in  different  form. 

It  will  be  our  first  task  to  discuss  the  rise  of  the  evolution 
theory  and  in  particular  its  relation  to  the  subject  of  genetics. 
Subsequently  we  shall  discuss  the  known  facts  of  genetics  and 
the  several  ways  in  which  biologists  interpret  them;  and 
finally  we  shall  discuss  human  evolution  as  a  subdivision  of 
genetics,  and  its  social  control,  or  eugenics. 


PART  I 

GENETICS 


CHAPTER  I 

DARWIN'S  THEORY  OF  EVOLUTION  AND 
ITS  EVIDENCES 

The  human  mind  is  characterized  above  all  else  by  curiosity, 
the  source  of  all  our  wisdom  as  well  as  of  our  woes.  This  fact 
the  ancients  portray  in  the  tale  of  Pandora's  box.  We  in- 
stinctively seek  an  explanation  of  all  the  phenomena  of 
nature,  unless  our  natural  curiosity  has  been  repressed  by 
convention  or  education  (falsely  so  called).  We  demand  a 
reason  for  everything,  and  if  none  is  forthcoming  from  an  out- 
side source,  we  straightway  construct  one  for  ourselves  out  of 
our  own  imaginings.  This  is  the  attitude  of  mind  of  the  child 
whose  perpetual  "  why  "  and  "  what  "  are  so  distressing  to 
perplexed  parents.  It  is  the  attitude  of  mind  in  which  all 
primitive  peoples  and  original  thinkers  have  regarded  the 
phenomena  of  nature.  It  was  this  attitude  of  mind  which  led 
to  the  formulation  of  the  evolution  theory,  which  is  an  attempt 
to  explain  the  present  condition  of  the  world  in  terms  of  simpler 
pre-existing  conditions. 

When  evolution  is  mentioned,  we  think  of  Darwin  as  its 
originator,  but  in  reality  he  did  not  originate  it;  the  idea  of 
organic  evolution  had  often  been  suggested  before  his  time, 
but  he  proved  its  reality.  The  principle  of  evolution  had 
long  been  recognized  in  relation  to  inorganic  things.  In 
chemistry,  physics,  and  astronomy,  the  constancy  and  inde- 
structibility of  matter  were  fully  established.  It  was  recog- 
nized for  example  that  more  complex  states  of  matter,  that  is, 
"  chemical  compounds,"  may  arise  out  of  the  simpler  "  ele- 
ments "  by  their  combination  in  definite  proportions,  and 
that  out  of  such  compounds  the  elements  may  by  suitable 
means  be  recovered  again  unchanged  and  in  the  original  pro- 
portions. 

7 


8  GENETICS  AND  EUGENICS 

In  geology,  the  work  of  Lyell  had  shown  that  the  present 
condition  of  the  earth's  crust  had  come  about  gradually 
through  the  action  of  causes  still  at  work. 

Accordingly  in  all  the  fundamental  sciences  which  deal 
with  the  inorganic  world  the  reign  of  natural  law  was  ac- 
knowledged before  the  time  of  Darwin,  and  the  principle  of 
miraculous  change  was  no  longer  offered  as  an  explanation  of 
existing  conditions. 

But  in  the  realm  of  living  things  it  was  in  Darwin's  time 
very  different.  The  animal  kingdom  was  not  supposed  to 
have  grown,  but  to  have  been  made  outright.  The  higher 
animals  were  not  supposed  to  have  originated  from  lower 
ones  but  to  have  been  made  in  the  form  in  which  they  exist 
today.  It  was  Darwin's  work  which  dispelled  this  outgrown 
idea,  and  established  the  principle  of  evolution  as  an  explana- 
tion of  the  organic  as  well  as  of  the  inorganic  world.  In  his 
time  the  idea  was  so  novel  as  applied  to  animals  and  plants 
that  it  aroused  the  greatest  opposition.  But  the  idea  was  not 
wholly  new  to  human  thought;  in  forms  more  or  less  fanciful 
and  incomplete  it  had  been  suggested  in  previous  centuries 
from  the  days  of  the  early  Greek  philosophers  on.  ^ 

Darwin  lived  in  a  time  peculiarly  inhospitable  to  the  idea 
of  organic  evolution,  partly  because  of  theological,  and  partly 
because  of  scientific  dogma.  Had  the  idea  been  brought  for- 
ward centuries  before  accompanied  by  proofs  such  as  Darwin 
advanced  in  its  support,  it  undoubtedly  would  have  met  more 
ready  acceptance  than  it  found  in  the  last  century.  As  it 
was,  Darwin  had  to  make  the  discovery  anew  for  himself, 
largely  unaided  by  his  predecessors,  who,  though  they  had 
formulated  more  or  less  clearly  the  same  line  of  explanation 
which  he  adopted,  had  failed  to  put  it  to  the  test  of  long- 
continued  and  detailed  observation  and  experiment,  which 
alone  sufficed  firmly  to  establish  it. 

1  Professor  H.  F.  Osborn  ('94)  has  described  in  a  most  interesting  book  the 
various  foreshadowings  of  the  idea  of  organic  evolution  which  appear  in  the  writings 
of  Darwin's  predecessors,  and  the  development  of  the  idea  in  Darwin's  own  mind 
as  evidenced  by  his  letters  and  other  writings.  One  interested  in  the  historical  and 
philosophical  growth  of  the  idea  cannot  do  better  than  to  consult  Osborn's  book. 


DARWIN'S  LIFE  9 

Charles  Darwin  was  born  in  1809  and  died  in  1882.  Both 
his  father  and  his  paternal  grandfather  were  physicians;  the 
grandfather,  Erasmus  Darwin,  was  also  a  naturalist  and  phi- 
losopher of  note,  who  anticipated  many  of  the  evolutionary 
ideas  of  Lamarck  and  some  of  those  of  his  own  illustrious 
grandson. 

On  his  mother's  side,  Darwin's  grandfather  was  Josiah 
Wedgwood,  the  famous  manufacturer  of  pottery.  Francis 
Galton,  the  founder  of  Eugenics,  was  his  cousin.  Those  who 
consider  special  tastes  and  talents  hereditary  find  significance 
in  these  relationships.  Thus  one  biographer,  after  noting 
that  Darwin's  father  had  originally  intended  him  for  the 
Church,  continues  "but  hereditary  tendencies  toward  nat- 
ural history  led  him  in  another  direction."  It  may  fairly  be 
questioned  whether  **  tendencies  toward  natural  history  " 
are  hereditary  in  the  strict  sense  of  the  word  any  more  than 
tendencies  toward  pottery,  which  Darwin  does  not  seem  to 
have  manifested  though  his  grandfather  was  Josiah  Wedg- 
wood. Such  language  as  I  have  quoted  is  quite  permissible 
on  the  part  of  a  literary  biographer  (indeed  Darwin  speaks  in 
like  vein  in  his  autobiography)  but  the  student  of  eugenics 
must  be  on  his  guard  against  accepting  it  at  its  face  value. 

What  Darwin  probably  inherited  was  not  a  "  tendency 
toward  natural  history  "  but  a  good  mind;  what  subjects 
engaged  it  was  probably  determined  not  by  inheritance  but 
by  the  subjects  which  came  to  his  attention  at  the  period  of 
life  when  men  do  their  best  creative  thinking.  In  Darwin's 
case,  the  thing  which  centered  his  attention  upon  the  prob- 
lem of  the  origin  of  species  and  held  it  there  for  the  rest  of 
his  lifetime  was  the  famous  voyage  of  the  Beagle. 

In  school  Darwin  was  not  a  distinguished  student.  He 
attended  Edinburgh  University  for  two  sessions  and  then  the 
University  of  Cambridge,  where  he  took  the  B.A.  degree  in 
1831.  Shortly  after  graduation  he  seized  the  opportunity  to 
go  as  naturalist  on  the  ship  Beagle  of  the  English  navy,  which 
was  detailed  on  a  voyage  of  exploration  round  the  world. 
This  voyage  lasted  almost  five  years,  from  December  27, 


10  GENETICS  AND  EUGENICS 

1831,  to  October  2,  1836.  Much  time  was  spent  by  this 
expedition  in  making  surveys  of  southern  South  America, 
ana  of  oceanic  islands.  For  a  large  part  of  this  time  Darwin 
was  brought  into  intimate  daily  contact  with  the  animals 
and  plants  of  an  unexplored  part  of  the  world.  What  a  post- 
graduate course  in  natural  history  this  was!  It  is  probably 
fortunate  that  his  previous  studies  of  natural  history  had  not 
been  more  specialized  and  detailed,  and  that  he  had  no  master 
at  hand  to  guide  him  in  his  studies  during  the  voyage.  Other- 
wise he  would  certainly  have  been  hampered  by  precon- 
ceived ideas  and  have  been  less  inclined  to  depart  from  ac- 
cepted notions.  But  here  he  was  face  to  face  with  a  new  world 
of  animals  and  plants  awaiting  explanation,  and  his  it  was  to 
study  them  without  assistance  or  let  up  for  three  years.  For 
an  ordinary  boy  of  twenty-two,  what  a  perplexing  and  be- 
wildering task,  what  a  fate,  sentenced  to  five  years  of  sea- 
sickness, the  effects  of  which  were  to  last  throughout  his  life! 
But  for  a  Darwin,  what  an  opportunity,  to  study  at  first  hand 
the  animals,  the  plants,  the  peoples  of  all  lands  and  of  all 
seas! 

After  Darwin  had  spent  some  three  years  on  the  Beagle  he 
returned  home  with  impaired  health  which  forced  him  to  live 
quietly  at  his  country  home  in  Downs,  England.  Here  he 
devoted  a  part  of  each  day  to  working  up  the  scientific  results 
of  his  journey,  and  published  during  the  next  twenty  years  an 
attempt  to  correlate,  to  unify  and  to  explain  the  various  ob- 
servations which  he  had  made,  an  attempt  which  finally 
found  fruition  in  his  theory  of  evolution  through  natural 
selection. 

It  had  long  been  known  to  a  number  of  Darwin's  scientific 
friends  that  he  was  working  on  a  theory  of  evolution  when, 
in  1858,  he  received  from  A.  R.  Wallace,  then  in  the  East 
Indies,  the  manuscript  of  a  paper  containing  precisely  the 
same  explanation  of  organic  adaptations  which  he  himself 
had  reached.  Darwin  was  naturally  much  embarrassed,  but 
seemed  willing  to  throw  aside  his  own  work  and  give  prece- 
dence to  Wallace's  paper.    On  the  advice  of  friends,  however. 


DARWIN'S  THEORY  11 

he  submitted  to  the  Linnaean  Society  of  London  an  abstract 
of  his  own  conclusions,  which  was  read  and  pubhshed  simul- 
taneously with  the  paper  by  Wallace.  The  work  of  each 
author  was  so  manifestly  independent  of  the  other  and  each 
dealt  so  generously  with  the  other  that  no  rivalry  arose 
between  them,  and  both  were  to  the  last  the  best  of  friends. 
The  essential  points  in  their  theory,  which  Darwin  elabor- 
ated more  fully  the  following  year  (1859)  in  his  Origin  of 
Species,  have  been  summarized  thus  by  Conn  (p.  353)  : 

"  1.  Overproduction,  All  animals  and  plants  tend  to 
multiply  more  rapidly  than  it  is  possible  for  them  to  continue 
to  exist.  More  offspring  are  produced  by  even  the  slowest 
breeding  animals  and  plants  than  can  possibly  find  susten- 
ance in  the  world. 

"  2.  Struggle  for  existence.  As  a  result  of  overproduction, 
the  individuals  that  are  born  are  engaged  in  a  constant 
struggle  with  each  other  for  the  opportunity  to  live.  This 
struggle  is  sometimes  an  active,  sometimes  a  passive  one; 
and  sometimes  it  is  a  struggle  with  each  other  for  food.  It 
is  a  struggle  in  which  only  the  victors  remain  alive,  the 
vanquished  being  exterminated  without  living  long  enough 
to  leave  offspring. 

"  3.  Variation,  or  diversity.  All  animals  and  plants  show 
a  large  amount  of  diversity  among  themselves,  and,  as  a 
result,  some  must  be  better  fitted  for  the  struggle  for  life 
than  others. 

''4.  Natural  selection,  or  the  survival  of  the  fittest.  It  is  a 
logical  result  of  the  struggle  for  existence  that  only  those 
individuals  best  fitted  for  the  struggle  will  be  the  ones,  in 
the  long  run,  to  win  in  the  contest.  Hence  the  "  fittest " 
in  the  long  run  will  survive,  while  those  less  fitted  to  exist 
will  be  exterminated. 

"5.  Heredity.  By  the  laws  of  heredity,  individuals  trans- 
mit to  their  offspring  their  own  characters.  Hence  if  one 
individual  survives  the  struggle  for  existence  by  virtue  of 
some  special  characteristic,  it  will  transmit  this  characteristic 
to  its  offspring.     The  offspring  will  inherit  it,  and  in  the 


12  GENETICS  AND  EUGENICS 

course  of  a  few  generations  the  only  individuals  left  alive 
will  be  those  that  have  developed  it,  while  those  that  did 
not  develop  it  will  be  exterminated  by  the  law  of  natural 
selection." 

This  theory  stands  today  in  the  main  as  Darwin  left  it, 
the  chief  advances  since  his  time  being  concerned  with  one 
or  other  of  the  two  factors,  variation  and  heredity,  concern- 
ing which  our  knowledge,  though  still  incomplete,  has  made 
notable  advances.  But  before  we  pass  to  the  consideration 
of  these,  let  us  pause  to  inquire  what  were  the  lines  of 
evidence  upon  which  Darwin  relied  to  establish  his  theory. 

These  have  been  well  summarized  by  T.  H.  Huxley  (1825- 
1895)  who  by  his  able  championship  of  Darwin's  views  did 
more  than  any  other  one  man  to  gain  for  these  views  general 
recognition  and  acceptance.  As  modified  by  Lock,  Huxley's 
summary  is  as  follows :  — 

"  1.  The  Gradation  of  Organisms,  Both  in  the  animal  and 
vegetable  kingdoms  we  may  trace,  in  spite  of  certain  gaps,  a 
long  series  of  gradations  in  complexity  of  structure,  so  that 
between  the  simplest  and  the  most  complicated  of  living 
things  a  great  number  of  intermediate  stages  are  to  be  found. 
When  we  pass  to  the  lower  end  of  the  scale  in  either  case,  we 
come  upon  a  group  of  creatures  of  comparatively  simple 
organization.  Among  them  we  find  members  with  regard 
to  which  we  cannot  definitely  say  that  they  are  either  animals 
or  plants.  Moreover,  these  unicellular  organisms  resemble 
in  many  ways  the  egg-cell  from  which  every  individual  among 
the  higher  animals  and  plants  originates. 

"2.  Embryology.  All  the  members  of  a  particular  group  of 
animals  or  plants  as  a  rule  resemble  one  another  more  closely 
in  the  early  stages  of  their  individual  development  than  they 
do  in  the  adult  condition,  and  in  the  earliest  stages  of  all  they 
are  often  indistinguishable.  These  facts  are  explained  if  we 
suppose  that  such  individuals  have  a  common  origin,  that 
they  are  descended  from  a  common  ancestor,  and  that  traces 
of  their  pedigree  are  still  to  be  observed  in  the  developmental 
stages  through  which  each  one  passes.    We  do  not  find  a  com- 


DARWIN'S  EVIDENCES  13 

plete  parallelism  between  the  development  of  the  individual 
and  the  history  of  the  race,  nor  should  we  expect  to  do  so, 
since  embryonic  as  well  as  adult  stages  may  be  modified  in  the 
course  of  evolution;  what  we  should  expect  is  a  more  or  less 
vague  historical  sketch,  and  this  is  what  is  usually  found 
remaining. 

"3.  Morphology.  On  comparing  together  the  different 
members  of  one  of  the  great  groups  or  classes  of  animals  or 
plants,  we  find  the  same  fundamental  plan  of  organization 
running  through  all  of  them.  Series  of  corresponding  organs 
are  often  to  be  made  out  which  are  built  upon  the  same 
general  scheme,  although  their  functions  may  be  quite  dis- 
similar; so  that,  for  instance,  in  the  hand  of  a  man,  the  paw 
of  a  dog,  the  wing  of  a  bat,  and  the  paddle  of  a  whale,  almost 
identically  the  same  series  of  bones  can  be  traced.  An  ob- 
vious explanation  is  to  be  found  in  the  supposition  that  these 
parts  have  arisen  by  the  divergent  modification  of  parts 
which  were  originally  identical. 

"4.  Geographical  Distribution.  Observation  shows  that 
groups  of  closely  allied  creatures  are  often  found  living  in 
neighbouring  districts,  and  that  when  such  a  barrier  as  an 
ocean  or  a  range  of  lofty  mountains  is  passed  an  entirely  new 
fauna  and  fiora  are  usually  to  be  met  with.  These  facts  may 
be  explained  by  the  hypothesis  that  allied  groups  of  species 
originated  by  a  process  of  descent  in  the  same  countries  which 
they  now  inhabit,  and  they  can  be  explained  by  no  other 
known  hypothesis. 

"  5.  The  Geological  Succession  of  Organisms.  The  general 
facts  regarding  the  distribution  of  allied  species  of  animals 
and  plants  in  time  point  in  precisely  the  same  direction  as 
those  relating  to  their  distribution  in  space.  In  a  few  cases, 
notably  in  that  of  the  extinct  horse  of  North  America,  a  long 
chain  of  possibly  ancestral  types  has  been  found  leading  back 
to  a  remote  and  very  different  progenitor.  This  supposed 
ancestor  of  the  horse  was  a  creature  little  larger  than  a 
moderate-sized  dog.  It  had  four  separate  toes  to  each  fore- 
limb,  and  three  to  each  hind-limb,  and  its  teeth  were  much 


14  GENETICS  AND  EUGENICS 

simpler  and  less  specialized  than  those  of  existing  horses. 
The  general  distribution  of  organisms  throughout  the  geo- 
logical strata  agrees,  moreover,  in  a  remarkable  way  with 
what  is  to  be  expected  on  the  evolution  theory. 
^  "6.  Changes  under  Domestication.  Among  domesticated 
animals  and  plants  we  know  of  numerous  cases  in  which  the 
actual  origin  of  new  forms  has  been  observed.  These  have 
often  differed  from  their  predecessors  by  amounts  quite  com- 
parable with  the  differences  by  which  natural  species  or  even 
genera  are  separated.  A  notable  example  of  this  process  is 
afforded  by  the  numerous  breeds  of  pigeons  known  to  have 
arisen  under  domestication  from  a  single  wild  species.  We 
have  no  reason  whatever  for  supposing  that  domesticated 
species  are  more  mutable  than  wild  species,  and  there  is  con- 
sequently every  reason  to  believe  that  changes  of  a  similar 
character  take  place  in  Nature.  The  conditions  of  domesti- 
cation, of  course,  afford  much  better  opportunities  of  observ- 
ing such  phenomena. 

\  "  7.  The  Observed  Facts  of  Mutation.  Nevertheless,  indi- 
vidual specimens  of  particular  wild  species  are  frequently 
found  showing  modifications  which,  if  they  occurred  con- 
stantly in  an  isolated  group,  would  afford  a  basis  for  the 
description  of  new  species.  In  a  few  cases  the  actual  occur- 
rence of  similar  changes  has  been  observed  in  wild  species  of 
plants. 

"  We  see,  therefore,  that  the  evidence  in  favour  of  the 
existing  species  of  animals  and  plants,  having  arisen  by  a 
process  of  evolution,  is  of  a  most  ample  and  convincing  kind." 

How  some  of  these  evidences  first  presented  themselves 
to  Darwin's  mind  and  how  he  came  later  to  value  them,  Dar- 
win states  in  the  closing  pages  of  the  Introduction  to  his 
Variation  of  Animals  and  Plants  under  Domestication, 

WTien  I  visited,  during  the  voyage  of  H.  M.  S.  Beagle,  the  Galapagos 
Archipelago,  situated  in  the  Pacific  Ocean  about  five  hundred  miles  from 
South  America,  I  found  myself  surrounded  by  peculiar  species  of  birds, 
reptiles,  and  plants,  existing  nowhere  else  in  the  world.  Yet  they  nearly 
all  bore  an  American  stamp.  In  the  song  of  the  mocking-thrush,  in  the 
harsh  cry  of  the  carrion-hawk,  in  the  great  candlestick-like  opuntias,  I 


DARWIN'S  OWN  ACCOUNT  15 

clearly  perceived  the  neighbourhood  of  America,  though  the  islands  were 
separated  by  so  many  miles  of  ocean  from  the  mainland,  and  differed  much 
in  their  geological  constitution  and  climate.  Still  more  surprising  was  the 
fact  that  most  of  the  inhabitants  of  each  separate  island  in  this  small 
archipelago  were  specifically  different,  though  most  closely  related  to  each 
other.  The  archipelago,  with  its  innumerable  craters  and  bare  streams  of 
lava,  appeared  to  be  of  recent  origin;  and  thus  I  fancied  myself  brought 
near  to  the  very  act  of  creation.  I  often  asked  myself  how  these  many 
peculiar  animals  and  plants  had  been  produced:  the  simplest  answer 
seemed  to  be  that  the  inhabitants  of  the  several  islands  had  descended 
from  each  other,  undergoing  modification  in  the  course  of  their  descent; 
and  that  all  the  inhabitants  of  the  archipelago  were  descended  from  those 
of  the  nearest  land,  namely  America,  whence  colonists  would  naturally 
have  been  derived.  But  it  long  remained  to  me  an  inexplicable  problem 
how  the  necessary  degree  of  modification  could  have  been  effected,  and  it 
would  have  thus  remained  for  ever,  had  I  not  studied  domestic  productions, 
and  thus  acquired  a  just  idea  of  the  power  of  Selection.  As  soon  as  I  had 
fully  realized  tliis  idea,  I  saw,  on  reading  Malthus  on  Population,  that 
Natural  Selection  w^as  the  inevitable  result  of  the  rapid  increase  of  all 
organic  beings ;  for  I  was  prepared  to  appreciate  the  struggle  for  existence 
by  having  long  studied  the  habits  of  animals. 

Before  visiting  the  Galapagos  I  had  collected  many  animals  whilst 
travelling  from  north  to  south  on  both  sides  of  America,  and  everjy'where, 
under  conditions  of  life  as  different  as  it  is  possible  to  conceive,  American 
forms  were  met  with  —  species  replacing  species  of  the  same  peculiar 
genera.  Thus  it  was  when  the  Cordilleras  were  ascended,  or  the  thick 
tropical  forests  penetrated,  or  the  fresh  waters  of  America  searched.  Sub- 
sequently I  visited  other  countries,  which  in  all  their  conditions  of  life  were 
incomparably  more  like  parts  of  South  America,  than  the  different  parts 
of  that  continent  are  to  each  other;  yet  in  these  countries,  as  in  Australia 
or  Southern  Africa,  the  traveller  cannot  fail  to  be  struck  with  the  entire 
difference  of  their  productions.  Again  the  reflection  was  forced  on  me 
that  community  of  descent  from  the  early  inhabitants  of  South  America 
would  alone  explain  the  wide  prevalence  of  American  types  throughout 
that  immense  area. 

To  exhume  with  one's  own  hands  the  bones  of  extinct  and  gigantic 
quadrupeds,  brings  the  whole  question  of  the  succession  of  species  vividly 
before  one's  mind;  and  I  found  in  South  America  great  pieces  of  tesselated 
armour  exactly  like,  but  on  a  magnificent  scale,  that  covering  the  pigmy 
armadillo;  I  had  found  great  teeth  like  those  of  the  living  sloth,  and  bones 
like  those  of  the  cavy.  An  analogous  succession  of  allied  forms  had  been 
previously  observed  in  Australia.  Here  then  we  see  the  prevalence,  as  if 
by  descent,  in  time  as  in  space,  of  the  same  types  in  the  same  areas;  and 
in  neither  case  does  the  similarity  of  the  conditions  by  any  means  seem 
sufficient  to  account  for  the  similarity  of  the  forms  of  life.  It  is  notorious 
that  the  fossil  remains  of  closely  consecutive  formations  are  closely  allied 
in  structure,  and  w^e  can  at  once  understand  the  fact  if  they  are  closely 
allied  by  descent.    The  succession  of  the  many  distinct  species  of  the  same 


16  GENETICS  AND  EUGENICS 

genus  throughout  the  long  series  of  geological  formations  seems  to  have 
been  unbroken  or  continuous.  New  species  come  in  gradually  one  by  one. 
Ancient  and  extinct  forms  of  life  are  often  intermediate  in  character,  like 
the  words  of  a  dead  language  with  respect  to  its  several  offshoots  or  living 
tongues.  All  these  facts  seemed  to  me  to  point  to  descent  with  modifica- 
tion as  the  means  of  production  of  new  species. 

The  innumerable  past  and  present  inhabitants  of  the  world  are  con- 
nected together  by  the  most  singular  and  complex  affinities,  and  can  be 
classed  in  groups  under  groups,  in  the  same  manner  as  varieties  can  be 
classed  under  species  and  sub-varieties  under  varieties,  but  with  much 
higher  grades  of  difference.  These  complex  affinities  and  the  rules  for 
classification,  receive  a  rational  explanation  on  the  theory  of  descent,  com- 
bined with  the  principle  of  natural  selection,  which  entails  divergence  of 
character  and  the  extinction  of  intermediate  forms.  How  inexplicable  is 
the  similar  pattern  of  the  hand  of  a  man,  the  foot  of  a  dog,  the  wing  of  a 
bat,  the  flipper  of  a  seal,  on  the  doctrine  of  independent  acts  of  creation! 
How  simply  explained  on  the  principle  of  the  natural  selection  of  successive 
slight  variations  in  the  diverging  descendants  from  a  single  progenitor !  So 
it  is  with  certain  parts  or  organs  in  the  same  indi\'idual  animal  or  plant,  for 
instance,  the  jaws  and  legs  of  a  crab,  or  the  petals,  stamens,  and  pistils  of  a 
flower.  During  the  many  changes  to  which  m  the  course  of  time  organic 
beings  have  been  subjected,  certain  organs  or  parts  have  occasionally  be- 
come at  first  of  little  use  and  ultimately  superfluous;  and  the  retention  of 
such  parts  in  a  rudimentary  and  useless  condition  is  intelligible  on  the 
theory  of  descent.  It  can  be  shown  that  modifications  of  structure  are 
generally  inherited  bj^  the  offspring  at  the  same  age  at  which  each  succes- 
sive variation  appeared  in  the  parents;  it  can  further  be  shown  that  varia- 
tions do  not  commonly  supervene  at  a  very  early  period  of  embryonic 
growth,  and  on  these  two  principles  we  can  imderstand  that  most  wonder- 
ful fact  in  the  whole  circuit  of  natural  history,  namely,  the  close  similarity 
of  the  embryos  within  the  same  class  —  for  instance,  those  of  mammals, 
birds,  reptiles,  and  fish. 

It  is  the  consideration  and  explanation  of  such  facts  as  these  which  has 
convinced  me  that  the  theory  of  descent  with  modification  by  means  of 
natural  selection  is  in  the  main  true.  These  facts  as  yet  received  no  ex- 
planation on  the  theory  of  independent  Creation;  they  cannot  be  grouped 
together  under  one  point  of  view,  but  each  has  to  be  considered  as  an 
ultimate  fact.  As  the  first  origin  of  life  on  this  earth,  as  w^ell  as  the  con- 
tinued life  of  each  individual,  is  at  present  quite  beyond  the  scope  of 
science,  I  do  not  wish  to  lay  much  stress  on  the  greater  simplicity  of  the 
view  of  a  few  forms  or  of  only  one  form  having  been  originally  created, 
instead  of  innumerable  periods;  though  this  more  simple  view  accords 
well  with  Maupertuis's  philosophical  axiom  of  "  least  action." 

In  considering  how  far  the  theory  of  natural  selection  may  be  ex- 
tended; that  is,  in  determining  from  how  many  progenitors  the  inhabitants 
of  the  world  have  descended,  —  we  may  conclude  that  at  least  all  the  mem- 
bers of  the  same  class  have  descended  from  a  single  ancestor.  A  number 
of  organic  beings  are  included  in  the  same  class,  because  they  present. 


DARWIN'S  FINAL  VIEWS  17 

independently  of  their  habits  of  Hfe,  the  same  fundamental  type  of  struc- 
ture, and  because  they  graduate  into  each  other.  Moreover,  members  of 
the  same  class  can  in  most  cases  be  shown  to  be  closely  alike  at  an  early 
embryonic  age.  These  facts  can  be  explained  on  the  belief  of  their  descent 
from  a  common  form;  therefore  it  may  be  safely  admitted  that  all  the 
members  of  the  same  class  are  descended  from  one  progenitor.  But  as  the 
members  of  quite  distinct  classes  have  something  in  common  in  structure 
and  much  in  common  in  constitution,  analogy  would  lead  us  one  step 
further,  and  to  infer  as  probable  that  all  Hving  creatures  are  descended 
from  a  single  prototype. 

I  hope  that  the  reader  will  pause  before  coming  to  any  final  and  hostile 
conclusion  on  the  theory  of  natural  selection.  The  reader  may  consult  my 
"  Origin  of  Species  "  for  a  general  sketch  of  the  whole  subject;  but  in  that 
work  he  has  to  take  many  statements  on  trust.  In  considering  the  theory 
of  natural  selection,  he  will  assuredly  meet  with  weighty  difficulties,  but 
these  difficulties  relate  chiefly  to  subjects  —  such  as  the  degree  of  perfec- 
tion of  the  geological  record,  the  means  of  distribution,  the  possibility  of 
transitions  in  organs,  etc.,  on  which  we  are  confessedly  ignorant;  nor  do 
we  know  how  ignorant  we  are.  If  we  are  much  more  ignorant  than  is 
generally  supposed,  most  of  these  difficulties  wholly  disappear.  Let  the 
reader  reflect  on  the  difficulty  of  looking  at  whole  classes  of  facts  from  a 
new  point  of  view.  Let  him  observe  how  slowly,  but  surely,  the  noble 
views  of  Lyell  on  the  gradual  changes  now  in  progress  on  the  earth's  sur- 
face have  been  accepted  as  sufficient  to  account  for  all  that  we  see  in  its 
past  history.  The  present  action  of  natural  selection  may  seem  more  or 
less  probable;  but  I  believe  in  the  truth  of  the  theory,  because  it  collects, 
under  one  point  of  view,  and  gives  a  rational  explanation  of,  many  ap- 
parently independent  classes  of  facts. 

In  his  earlier  statements  of  his  theory,  Darwin  does  not 
seem  to  have  paid  much  attention  to  the  source  of  variations 
or  to  the  manner  of  their  inheritance,  but  these  subjects  re- 
ceive much  attention  in  his  great  work  on  the  Variation  of 
animals  and  plants  under  domestication,  from  which  we 
have  just  quoted.  He  seems  to  have  come  more  and  more  to 
hold  views  similar  to  those  of  Lamarck,  his  great  French  pre- 
decessor, regarding  the  direct  effect  of  environment  as  a  cause 
of  variation,  and  the  inheritance  of  effects  so  produced.  Con- 
cerning the  general  nature  of  Lamarck's  views  we  should 
therefore  inform  ourselves. 


CHAPTER  II 

CONTRIBUTIONS  OF  LAMARCK,  WEISMANN,  AND  HERBERT 
SPENCER  TO  THE  THEORY  OF  EVOLUTION;    DARWIN'S 

THEORY  OF  PANGENESIS 

Lamarck  (1744-1829),  the  greatest  evolutionist  before  Dar- 
win, was,  according  to  his  biographer,  a  man  of  great  physical 
and  moral  courage.  He  distinguished  himself  by  a  deed  of 
singular  bravery  in  the  French  army,  and,  receiving  an  in- 
jury, re-entered  life  as  a  doctor.  He  was  first  attracted  to 
botany  by  the  rich  flora  near  Monaco  observed  during  his 
military  service.  Going  to  Paris  he  gained  the  attention  of 
the  great  naturalist,  Buff  on,  under  whose  direction  he  pub- 
lished a  "  Flora  of  France,"  written  in  six  months,  which 
passed  through  many  editions.  He  seems  to  have  possessed 
powers  of  exceptionally  rapid  observation,  with  great  facility 
in  writing  and  with  unusual  powers  of  definition  and  descrip- 
tion. At  the  age  of  forty-nine  (1793)  he  was  transferred  to  a 
Zoological  chair  in  the  Jardins  des  Plantes,  being  placed  in 
charge  of  invertebrate  zoology,  while  at  the  same  time  Geoff- 
roy  Saint-Hilaire  was  placed  in  charge  of  vertebrate  zoology. 
Being  at  this  time  in  his  fiftieth  year,  Lamarck  took  up  the 
study  of  zoology  with  such  zeal  and  success  that  he  almost 
immediately  introduced  striking  reforms  in  classification,  and 
developed  (after  having  reached  middle  life)  the  conception 
of  the  mutability  of  species  and  of  the  origin  of  new  species 
by  descent.  His  relation  to  the  evolution  idea  was  thus  very 
different  from  Darwin's.  It  came  to  Darwin  almost  in  his 
boyhood  and  he  spent  a  lifetime  working  it  out,  not  publishing 
anything  upon  it  until  he  was  fifty  years  old.  To  Lamarck 
the  idea  seems  scarcely  to  have  come  before  the  age  of  fifty, 
and  he  rapidly  developed  it  into  a  system,  sufficiently  elabor- 
ate to  explain  evolution,  if  his  basic  principle  is  true,  viz. 

18 


LAMARCK  AND  HIS  THEORY  19 

the  inheritance  of  acquired  characters.    This  we  shall  consider 
further. 

Regarding  Lamarck's  later  life,  Osborn  (p.  158)  says: 

His  devotion  to  the  study  of  the  small  forms  of  life,  probably  with  in- 
ferior facilities  for  work,  for  he  was  extremely  poor,  gradually  deprived  him 
of  the  use  of  his  eyes,  and  in  1819  he  became  completely  blind.  The  last 
two  volumes  of  the  first  edition  of  his  Natural  history  of  invertehrated 
animals,  which  was  begun  in  1816  and  completed  in  1822,  was  carried  on 
by  dictation  to  his  daughter,  who  showed  him  the  greatest  devotion;  after 
Lamar.ck  was  confined  to  his  room,  it  is  said  she  never  left  the  house. 
Lamarck  was  thus  saddened  in  his  old  age  by  extreme  poverty  and  by  the 
harsh  reception  of  his  transmutation  theories,  in  the  truth  of  which  he 
felt  the  most  absolute  conviction. 

Lamarck's  Theory 

The  factors  recognized  by  Lamarck  as  concerned  in  evolu- 
tion may  be  summarized  as  follows :  — 

1.  The  direct  effect  of  environment.  We  know  that  a  plant 
in  rich  soil  grows  large  and  luxuriant,  but  that  the  same  plant 
in  poor  soil  would  remain  small  and  stunted.  This  is  a  direct 
effect  of  the  environment.  Lamarck  supposed  that  such 
effects  of  environment  are  cumulative  from  generation  to 
generation  so  that  long-continued  growing  in  rich  soil  would 
produce  a  more  luxuriant  race,  while  continued  growing  in 
poor  soil  would  produce  a  different  and  smaller  race.  In  the 
case  of  animals  Lamarck  does  not  think  that  the  action  of 
environment  is  quite  so  direct,  but  that  animals  are  changed 
indirectly  through  changes  in  their  habits.  Buff  on  considered 
the  action  of  environment  direct  in  both  animals  and  plants, 
and  this  view  Darwin  seems  to  have  adopted  rather  than 
Lamarck's  slightly  different  one.  Darwin  in  his  Variation 
adopts  this  factor,  the  direct  effect  of  environment,  as  one  of 
the  causes,  if  not  the  chief  cause  of  variations.  He  says 
(p.  6): 

If  then  organic  beings  in  a  state  of  nature  vary  even  in  a  slight  degree, 
owing  to  changes  in  the  surrounding  conditions,  of  which  we  have  abundant 
geological  evidence,  or  from  any  other  cause,  —  then  the  severe  and  often- 
recurrent  struggle  for  existence  will  determine  that  those  variations,  how- 
ever slight,  which  are  favorable  shall  be  preserved  or  selected,  and  those 
which  are  unfavorable  shall  be  destroyed. 


20  GENETICS  AND  EUGENICS 

2.  Lamarck  regarded  new  physical  needs  as  a  second  factor 
or  cause  of  variations.  He  supposed  that  the  need  of  an 
organ  caused  the  organ  to  be  produced,  that  need  of  horns  to 
fight  with  or  of  teeth  to  chew  with  would  cause  the  produc- 
tion of  horns  and  teeth  respectively.  Darwin  never  adopted 
this  view. 

3.  A  third  Lamarckian  factor  however  Darwin  did  regard 
as  a  genuine  cause  of  variation,  viz.,  use  and  disuse.  The  use 
of  an  organ,  as  the  arm  or  leg,  causes  it  to  increase  in  size 
and  strength;  conversely  disuse  causes  decrease  in  size  and 
efficiency. 

4.  Inheritance  of  acquired  characters.  As  regards  heredity, 
Lamarck  believed  that  variations  of  every  sort  are  inherited. 
Those  which  result  from  direct  action  of  the  environment  or 
from  use  and  disuse,  we  now  call  acquired  characters,  and 
Lamarck  supposed  that  acquired  characters  are  inherited.  In- 
deed he  supposed  that  all  variations  are  of  this  nature.  Dar- 
win shared  Lamarck's  view  in  part;  he  too  probably  did  not 
clearly  distinguish  between  variations  which  we  should  class 
as  acquired  characters  and  those  of  other  sorts.  Certainly 
Lamarck  did  not  make  this  distinction,  for  on  his  view  all 
variations  are  what  we  should  call  acquired. 

In  illustration  of  Lamarck's  views  concerning  the  causes  of 
variations  and  of  consequent  evolution,  it  may  be  well  to 
quote  a  few  passages  largely  in  his  own  words,  as  given  in 
translation  in  Osborn,  pp.  164-171. 

In  considering  the  natural  order  of  animals,  the  very  positive  gradation 
which  exists  in  their  structure,  organization,  and  in  the  number  as  well  as 
in  the  perfection  of  their  faculties,  is  very  far  removed  from  being  a  new 
truth,  because  the  Greeks  themselves  fully  perceived  it;  but  they  were  un- 
able to  expose  the  principles  and  the  proofs  of  this  evolution,  because  they 
lacked  the  knowledge  necessary  to  establish  it.  In  consideration  of  this 
gradation  of  life,  there  are  only  two  conclusions  which  face  us  as  to  its  ori- 
gin :  —  The  conclusion  adopted  up  to  today  :  Nature  (or  its  Author)  in  cre- 
ating animals  has  foreseen  all  possible  sorts  of  circumstances  in  which  they 
would  be  destined  to  live,  and  has  given  to  each  species  a  constant  organi- 
zation, as  well  as  a  form  determined  and  invariable  in  its  parts,  which  forces 
each  species  to  live  in  the  places  and  climates  where  it  is  found,  and  there 
to  preserve  the  habits  which  we  know  belong  to  it.  My  personal  conclusion: 
Nature,  in  producing  successively  all  the  species  of  animals,  and  commenc- 


LAMARCK,  DARWTN,  AND  WEISMANK  21 

ing  by  the  most  imperfect  or  the  most  simple  to  conchide  its  labour  in  the 
most  perfect,  has  gradually  completed  their  organization  ;  and  of  these 
animals,  while  spreading  generally  in  all  the  habitable  regions  of  the  globe, 
each  species  has  received,  under  the  influence  of  environment  which  it  has 
encountered,  the  habits  which  v/e  recognize  and  the  modifications  in  its 
parts  which  observation  reveals  in  it. 

All  that  Nature  has  caused  individuals  to  acquire  or  lose  by  the  influ- 
ences of  environment  to  which  they  have  been  long  exposed,  and  conse- 
quently by  the  influence  of  the  predominant  employment  of  a  certain  organ, 
or  by  that  of  the  continued  lack  of  use  of  the  same  part,  —  all  this  Nature 
conserves  by  generation  to  the  new  individuals  which  arise,  provided  that 
these  acquired  variations  (changements)  are  common  to  both  sexes,  or  to 
those  which  have  produced  these  new  individuals. 

But  great  changes  in  environment  bring  about  changes  in  the  habits  of 
animals.  Changes  in  their  wants  necessarily  bring  about  parallel  changes 
in  their  habits.  If  new  wants  become  constant  or  very  lasting,  they  form 
new  habits,  the  new  habits  involve  the  use  of  new  parts,  or  a  different  use 
of  old  parts,  which  results  finally  in  the  production  of  new  organs  and  the 
modification  of  old  ones. 

Darwin's  later  views  concerning  variation  and  heredity, 
as  compared  with  those  of  Lamarck,  may  be  briefly  stated 
thus: 

1.  Variation  was  thought  to  be  due  either  to  the  two 
Lamarckian  factors,  direct  action  of  the  environment  and  use 
or  disuse,  or  to  other  as  yet  unknown  causes,  the  results  of 
which  Darwin  refers  to  as  "  chance  variations." 

2.  As  regards  heredity,  Darwin  seems  to  have  thought 
with  Lamarck  that  variations  of  all  sorts  are  inherited, 
though  some  doubtless  were  inherited  more  strongly  and  per- 
sistently than  others. 

Weismann  (1834-1914).  The  first  great  advance,  after 
Darwin,  in  our  knowledge  of  variation  and  heredity  was  made 
by  Weismann,  a  German  zoologist,  who  within  two  years  after 
Darwin's  death  (viz.  in  1883)  brought  forward  a  new  classifi- 
cation of  variations  and  a  new  theory  of  heredity. 

He  showed  that  some  variations  are  congenital  {i.  e.,  are 
horn  with  us),  are  in  the  blood  so  to  speak,  while  others  are 
acquired  through  the  action  of  environment,  use  or  disuse. 
Regarding  acquired  characters,  he  showed  that  these,  in  all 
probability,  are  not  inherited.  This  was  a  wholly  new  idea 
and  called  forth  a  hot  debate  which  has  not  yet  ended,  but 


22 


GENETICS  AND  EUGENICS 


gradually  biologists  have  been  coming  to  the  view  that  Weis- 
mann  is  right.  The  consequences  of  this  view  are  very  im- 
portant not  only  as  regards  evolution  in  general,  but  also  as 
regards  education,  for  if  Weismann  is  right  scholarship  is  not 
inherited,  but  only  capacity  to  learn.  The  son  must  begin  in 
his  education,  not  where  his  father  left  off,  but  at  the  alpha- 
bet, and  he  will  not  learn  any  faster  because  his  father  was 
educated.  I  think  the  experience  of  educators  justifies  this 
view.  Children  growing  up  in  cultured  homes  have  a  certain 
educational  advantage  due  to  their  environment,  but  not  to 
heredity.  Thus  Darwin's  attention  was  directed  toward  nat- 
ural history,  by  the  home  environment  in  which  he  grew  up. 
The  same  is  true  in  even  greater  degree  of  his  sons,  three  of 
whom  have  become  distinguished  scientists.  It  is  very  im- 
probable that  he  inherited  a  taste  for  natural  history,  as  he 
supposed.    More  likely  he  acquired  such  a  taste. 


©G 

Fig.  1.    Diagram  showing  the  relation  of  the  body  or  soma  (S)  to  the  germ-cells 

(G)  in  heredity.    (After  E.  B.  Wilson.) 

Besides  showing  that  there  is  no  sufficient  evidence  that 
acquired  characters  are  inherited,  Weismann  pointed  out 
anatomical  and  physiological  reasons  why  we  should  not 
expect  them  to  be  inherited.  In  the  higher  animals  and 
plants  reproduction  takes  place  not  by  division  of  the  body 
but  by  the  development  of  special  reproductive  cells,  eggs, 
spores,  and  the  like.  The  fertiHzed  egg-cell  of  an  animal  be- 
gins its  development  by  dividing  into  two  cells;  these  divide 
into  four,  and  so  on.  Sooner  or  later  we  notice  that  these  cells 
are  not  all  alike.  Some  of  them  develop  into  muscles,  others 
into  bone,  or  nervous  tissue;  in  short  they  become  differ- 
entiated to  form  the  various  parts  and  tissues  of  the  body,  all 
except  some  few  which  remain  undifferentiated  like  the  origi- 
nal egg-cell  itself.    These  undifferentiated  cells  will  in  fact 


Fig.  2 


Fig.  3 


Fig.  4 


Fig.  5 


Fig.  6 


Fig.  7 

Results  of  ovarian  transplantation  in  guinea-pigs.  Ovaries  from  a  small  black  guinea-pig  (Fig.  •i) 
ivere  transplanted  into  an  albino  (Fig.  3)  which,  mated  with  another  albino  (Fig.  4),  produced  black 
young  (Figs.  5-7). 


BODY  AND  GERM-CELLS 


23 


give  rise  to  egg-cells  or  sperm-cells  rather  than  to  muscle, 
bone,  or  any  other  part  of  the  body  proper.  Weismann  called 
the  cells  which  collectively  make  up  the  body  the  soma  (Greek 
for  body) ;  whereas  those  undifferentiated  cells  destined  for 
reproduction  he  called  germ-cells  or  collectively  the  germ- 


Fig.  8.  Fruits  of  an  apple  "  graft-hybrid  "  or  "  chimera."  Two  distinct  varieties  are  represented  in 
one  fruit.  The  stem-end  of  the  apple  is  russet  and  sour;  the  blossom-end  is  smooth-skinned,  red-striped 
and  sweet.  A  sharp  line  of  division  separates  the  two  portions.  Such  fruits  are  borne  on  a  tree  pro- 
duced by  grafting  one  variety  on  another,  the  tree-trunk  having  grown  from  a  bud  which  arose  just 
where  stock  and  scion  join,  and  which  included  cells  derived  from  both  sources.  But  the  two  kinds 
of  cells  and  all  their  descendants  have  retained  their  original  distinctness,  as  the  composite  fruits  •ehow. 
Hence,  not  only  may  the  body  and  germ-cells  be  of  unlike  character  (as  Figs.  2-7  show),  but  even  the 
body  may  be  composite  and  yet  each  part  retain  its  original  character.  By  grafting  tadpoles,  Harrison 
has  produced  a  frog  which  anteriorly  was  of  one  species  and  posteriorly  of  another.  If  such  a  frog 
produced  eggs,  their  character  would  depend  upon  which  part  of  the  body  furnished  the  eggs.  "  Graft- 
hybrids  "  between  the  tomato  and  black  nightshade  (Solanum  nigrum)  produced  by  Winkler  and 
studied  by  him  and  by  Baur  were  found  to  produce  as  seedlings  either  pure  tomato  plants  or  pure 
nightshade  plants,  depending  on  which  species  made  up  that  part  of  the  "  chimera  "  from  which  the 
germ-cells  arise. 

plasm.  Now  Weismann  maintained  that  the  germ-cells,  since 
they  are  not  descended  from  body-cells  but  only  from  the 
fertilized  egg-cell,  have  no  way  of  transmitting  body-modifi- 
cations, i.  e.,  acquired  characters.  The  germ-cells  are  guests 
in  the  body,  but  not  members  of  the  household.  They  feed 
at  the  common  table  but  have  no  share  in  the  other  activities 


24  GENETICS  AND  EUGENICS 

of  the  home,  and  are  themselves  unmodified  bv  those  activi- 
ties.  To  show  the  biological  soundness  of  Weismann's  con- 
clusion that  soma  and  germ-plasm  are  anatomically  and 
physiologically  distinct,  I  may  cite  an  experiment  performed 
by  Dr.  John  C.  Phillips  and  myself: 

A  female  albino  guinea-pig  (Fig.  3)  just  attaining  sexual  maturity  was 
by  an  operation  deprived  of  its  ovaries,  and  instead  of  the  removed  ovaries 
there  were  introduced  into  her  body  the  ovaries  of  a  young  black  female 
guinea-pig  (Fig.  2),  not  yet  sexually  mature,  aged  about  three  weeks.  The 
grafted  animal  was  now  mated  with  a  male  albino  guinea-pig  (Fig.  4). 
From  numerous  experiments  with  albino  guinea-pigs  it  may  be  stated 
emphatically  that  normal  albinos  mated  together,  without  exception,  pro- 
duce only  albino  young,  and  the  presumption  is  strong,  therefore,  that  had 
this  female  not  been  operated  upon  she  would  have  done  the  same.  She 
produced,  however,  by  the  albino  male  three  litters  of  young,  which  to- 
gether consisted  of  six  individuals,  all  black.  (See  Figs.  5-7.)  The  first  litter 
of  young  was  produced  about  six  months  after  the  operation,  the  last  one 
about  a  year.  The  transplanted  ovarian  tissue  must  have  remained  in  its 
new  environment  therefore  from  four  to  ten  months  before  the  eggs  at- 
tained full  growth  and  were  discharged,  ample  time,  it  would  seem,  for  the 
influence  of  a  foreign  body  upon  the  inheritance  to  show  itself  were  such 
influence  possible. 

Since,  then,  germ-cells  and  body  are  distinct,  heritable 
variations  cannot  have  their  origin  in  body-cells  but  only 
in  the  germ-plasm.  The  problem  of  evolution,  therefore,  on 
Weismann's  view,  becomes  this  —  how  are  changes  in  the 
germ-plasm  brought  about  ? 
Darwin's  theory  of  ^pangenesis. 

Before  Weismann's  time,  Darwin,  in  common  with  biolo- 
gists in  general,  had  come  to  recognize  that  the  germ-cells 
(i.  e.,  the  egg  and  sperm-cells)  are  the  sole  vehicles  of  inheri- 
tance. Darwin  therefore  realized  that  if  acquired  characters 
are  inherited,  as  everyone  then  supposed,  bodily  modifica- 
tions must  in  some  way  be  registered  in  the  germ-cells,  and 
he  framed  an  hypothesis  to  explain  how  this  could  come 
about.  This  hypothesis,  which  he  called  Pangenesis,  is  put 
forward  in  the  closing  chapters  of  his  book  on  Animals  and 
plants  under  domestication.  Darwin  himself  was  not  sure 
of  its  correctness  and  advanced  it  as  he  says  "  tentatively  " 
only.    We  are  very  sure  that  it  was  not  correct,  but  it  has  for 


SPENCER  AND  PANGENESIS  25 

us  an  historical  interest  because  it  had  much  influence  upon 
biological  investigation  and  theory  at  that  time  and  subse- 
quently. Logically,  Darwin's  theory  of  pangenesis  may  be 
regarded  as  a  modification  of  one  of  Herbert  Spencer's  specu- 
lations upon  biology. 

Herbert  Spencer  (1820-1903)  was  the  champion  of  evolu- 
tion from  the  standpoint  of  philosophy,  as  Huxley  was  from 
the  standpoint  of  comparative  anatomy  and  embryology. 
His  ideas  had  much  influence  on  the  development  of  evolu- 
tionary thought  down  to  our  own  time.  (See  Delage  and 
Goldsmith,  1912.)  Spencer  tried  to  explain  the  structure  of 
living  substance  (protoplasm)  in  harmony  with  the  chemical 
explanation  of  lifeless  substance  then  current.  He  supposed 
that  there  are  structural  units  of  protoplasm  comparable  with 
the  molecules  of  chemical  compounds,  each  kind  of  proto- 
plasm within  the  body  being  composed  of  a  different  kind  or 
kinds  of  units.    These  he  called  physiological  units. 

Darwin  adopting  this  same  line  of  thought,  but  with  a 
more  intimate  knowledge  of  the  facts  of  inheritance,  saw  that 
every  kind  of  physiological  unit  must  be  supposed  to  exist  in 
the  germ-cell,  since  out  of  the  germ-cell  an  entire  body  de- 
velops. In  his  theory  of  pangenesis,  he  supposes  that  every 
part  of  the  body  is  constantly  giving  off  its  particular  kinds 
of  units  into  the  blood,  just  as  a  fungus  gives  off  spores  into 
the  air.  These  given  off  units  Darwin  called  "  gemmules," 
or  little  buds.  He  supposed  further  that  these  gemmules  are 
carried  through  the  body  in  the  blood  stream,  and  accumu- 
late in  the  germ-cells,  in  which  they  multiply  as  the  germ-cell 
develops.  Thus  out  of  one  germ-cell  comes  an  entire  body 
with  its  various  parts,  because  each  part  was  represented  in 
the  germ  by  a  gemmule.  No  one  today  holds  this  theory,  as 
Darwin  stated  it,  but  the  underlying  idea  of  preformed  deter- 
mining particles  existing  in  the  germ-cell  reappears  a  little 
later  in  Weismann's  theory  of  heredity,  and  has  wide  accep- 
tance today  in  the  chromosome  theory  of  inheritance. 

We  shall  come  to  these  later,  but  for  the  present  let  us  go 
back  to  Darwin's  theory  of  pangenesis.    Darwin's  method  of 


26  GENETICS  AND  EUGENICS 

reaching  this  theory  was  inductive  and  beyond  criticism.  He 
first  collected  all  the  facts  obtainable  about  inheritance  and 
then  attempted  to  frame  an  hypothesis  which  would  account 
for  them  all,  which  would  bring  them  all  under  one  point  of 
view.  Where  he  erred  was  in  accepting  as  facts  some  things 
which  we  know  are  not  facts.  In  fitting  a  theory  to  them,  he 
framed  a  false  theory,  simply  because  the  assumed  facts  were 
false. 

Darwin's  cousin,  Francis  Galton,  showed  the  unsoundness 
of  pangenesis  by  a  simple  experiment.  He  reasoned  thus.  If, 
as  Darwin  assumes,  gemmules  circulating  in  the  blood  deter- 
mine the  character  of  the  germ-cells,  then  blood  of  one  animal 
transfused  into  blood-vessels  of  another  should  carry  into  the 
germ-cells  of  the  second  animal  gemmules  derived  from  the 
first  animal.  Consequently  offspring  subsequently  produced 
by  an  animal  into  which  blood  has  been  transfused  should 
show  characteristics  of  the  animal  from  which  the  blood  was 
taken.  Galton  performed  this  experiment  on  rabbits  but  with 
results  wholly  negative.  The  experiment,  however,  cannot  be 
regarded  as  altogether  conclusive  because  (1)  blood  trans- 
fused from  one  individual  to  another  probably  does  not  long 
persist,  but  is  replaced  by  new  blood  formed  by  the  individual 
into  which  transfusion  occurred.  Therefore  the  effects  of 
transfusion  would  at  most  be  of  short  duration.  (2)  Suppos- 
ing that  modifications  were  induced  in  the  germ-cells  by 
transfusion,  it  is  not  to  be  expected,  in  the  light  of  our  present 
knowledge,  that  such  modifications  would  in  all  cases  appear 
in  the  first  generation  offspring,  but  rather  in  the  second  or 
later  generations  of  offspring,  but  Galton  did  not  carry  the 
experiment  so  far.  Galton's  experiment  therefore  cannot  be 
regarded  as  a  complete  refutation  of  pangenesis,  but  such  a 
refutation  has  become  unnecessary  through  the  development 
of  biological  knowledge  along  other  lines. 

The  theory  of  pangenesis  was  an  attempt  to  explain  the 
mechanism  of  the  inheritance  of  acquired  characters.  If 
acquired  characters  are  not  inherited,  as  we  now  have  reason 
to  think,  the  hypothesis  of  pangenesis  is  unnecessary  and 


PANGENESIS  DISCARDED  27 

should  accordingly  be  discarded.  This  in  fact  is  what  has 
actually  happened.  The  theory  as  Darwin  stated  it  has  no 
supporters  at  present.  Those  who  now  hold,  in  a  modified 
form,  that  acquired  characters  are  inherited,  have  adopted 
other  ways  of  explaining  their  inheritance,  or  else,  with  De- 
lage,  admit  the  inadequacy  of  Darwin's  explanation  and 
state  that  no  satisfactory  substitute  has  yet  been  found,  but 
entertain  the  hope  that  one  will  yet  be  discovered. 


CHAPTER  III 

ARE  ACQUIRED  CHARACTERS  INHERITED  ? 

Evidence  from  ovarian  transplantation  experiments  with 
guinea-pigs  has  been  cited  to  show  that  body  and  germ-cells 
are  morphologically  and  physiologically  distinct  and  that 
germ-cells  may  be  lodged  in  a  foreign  body  during  their  de- 
velopment without  losing  their  distinctive  character.  But 
this  by  no  means  proves  that  germ-cells  are  immune  from 
modification  by  influences  which  reach  them  through  the 
body.  The  evidence  cited  is  negative  evidence.  It  creates  a 
presumption  against  the  inheritance  of  acquired  characters 
but  does  not  prove  a  universal  negative,  which  is  impossible. 
The  question  whether  acquired  characters  are  or  are  not  in- 
herited is  therefore  a  question  to  be  decided  only  by  the  care- 
ful weighing  of  evidence.  It  is  possible  that  some  categories 
of  supposed  acquired  characters  are  more  readily  capable  of 
an  alternative  interpretation  than  are  others.  Several  of 
these  may  now  be  discussed  briefly. 

1.  Mutilations,  It  is  now  all  but  universally  admitted 
that  somatic  modifications  due  to  mutilation  are  not  in- 
herited. Nevertheless  "  cases  "  are  from  time  to  time  re- 
ported, in  which  a  man  or  a  domesticated  animal  which  by 
accident  had  lost  a  limb  has  produced  offspring  similarly  de- 
fective. One  of  the  most  frequently  recurring  of  these  stories 
has  come  to  me  at  first  hand.  A  cat  which  had  accidentally 
lost  her  tail  gave  birth  to  kittens  part  of  which  were  short- 
tailed.  It  is  not  necessary  to  suppose  that  the  report  is  in- 
accurate. Certain  races  of  cats  are  naturally  short-tailed, 
and  a  cat  might  produce  offspring  short-tailed  by  inheritance 
quite  irrespective  of  any  injury  to  either  parent.  On  the 
other  hand  where  docking  of  the  tail  has  been  followed  up 
systematically  for  many  generations  and  on  a  large  scale,  as 
is  the  case  in  sheep,  no  racial  shortening  of  the  tail  is  observ- 
es 


ACQUIRED  CHARACTERS  29 

able.  Finally,  we  have  the  direct  experimental  evidence  of 
Weismann,  who  cut  off  the  tails  of  mice  for  nineteen  genera- 
tions in  succession  without  however  observing  any  inheri- 
tance of  the  mutilation.  We  have  also  the  evidence  furnished 
by  long-continued  mutilations  practiced  by  man  upon  his 
own  person,  such  for  example  as  tatooing  and  circumcision. 
The  effects  of  such  mutilations,  as  is  well  known,  are  not  in- 
herited in  the  slightest  degree. 

Notwithstanding  all  this  negative  evidence,  Semon,  who 
like  a  drowning  man  catches  at  every  straw,  cites  Kammerer 
as  having  recently  shown  that  a  soft-bodied  marine  animal 
(Ciona,  an  ascidian)  after  its  siphons  are  cut  off  regenerates 
new  ones  longer  than  normal,  and  he  maintains  that  the 
young  of  such  animals  have  siphons  of  abnormal  length.  In 
view  of  all  the  negative  evidence  furnished  by  other  animals 
this  case,  as  yet  incompletely  published,  seems  highly  im- 
probable. The  unsupported  claim  throws  more  light  upon 
the  credibility  of  Kammerer  as  a  witness  (and  he  has  brought 
forward  many  cases  in  recent  years)  than  upon  the  general 
question  of  the  inheritance  of  mutilations. 

2.  Congenital  diseases.  Cases  of  disease  acquired  by  a 
parent  and  by  him  transmitted  to  his  offspring  are  frequently 
reported.  But  all  these  cases  are  capable  of  other  explana- 
tions than  that  of  inheritance  of  an  acquired  character. 

(a)  In  some  cases  a  disease-producing  organism  may  be 
present  in  the  body  of  the  parent  and  may  pass  directly  into 
the  reproductive  cell.  Thus  in  silkmoths,  the  organism 
which  causes  "pebrine"  is  transmitted  as  an  infection  within 
the  egg,  as  Pasteur  showed.  The  same  is  true  of  Texas  fever 
in  cattle.  This  disease  is  caused  by  a  protozoon  which  is 
introduced  into  the  blood  of  cattle  by  a  tick  which  harbors 
the  disease.  The  protozoan  parasite  is  present  in  the  egg-cell 
of  the  tick,  so  that  the  young  tick  which  develops  out  of  such 
infected  eggs  cannot  fail  to  contain  the  parasite;  but  the 
disease  is  no  more  inherited  than  a  grain  of  sand  placed  within 
the  egg  would  be  inherited.  In  a  similar  way  in  man  syphilis 
may  be  transmitted,  but  it  is  in  no  true  sense  inherited.    Yet 


30  GENETICS  AND  EUGENICS 

the  practical  outcome  is  very  similar;  an  individual  once  in- 
fected with  syphilis  is  racially  condemned;  his  seed  is  as 
truly  bad  as  if  the  syphilis  germ  were  an  essential  part  of  the 
germinal  substance. 

(b)  The  intimate  relationship  of  parent  to  child  may  give 
unusual  opportunities  for  post-natal  infection,  as  in  the  case 
of  tuberculosis.  Thus  the  children  of  tuberculous  parents 
are  more  liable  to  infection  with  tuberculosis,  other  things 
being  equal,  than  the  children  of  non-tuberculous  parents. 
But  we  are  not  justified  for  that  reason  in  speaking  of  tuber- 
culosis as  hereditary.  It  is  probably  in  all  cases  acquired  by 
the  patient,  individually,  and  not  inherited.  Whether  some 
individuals  are  more  susceptible  than  others  is  a  wholly 
different  question.    Susceptibility  may  well  be  inherited. 

(c)  Just  as  a  disease-producing  organism  may  be  received 
into  the  egg  or  the  embryo  while  it  is  still  within  the  body  of 
the  mother,  so  chemical  substances  in  the  mother's  blood  may 
enter  the  egg  or  embryo  and  affect  its  subsequent  character. 
Thus  it  has  been  shown  that  in  guinea-pigs  immunity  ac- 
quired by  the  mother  (which  is  known  to  be  due  to  the 
presence  of  specific  substances  in  the  blood)  may  be  trans- 
mitted to  her  offspring,  though  the  father  has  no  such  in- 
fluence, the  reason  being  that  the  sperm-cell  is  too  small  to 
carry  an  effective  quantity  of  antitoxin,  i.  e.,  of  immunity 
producing  substance.  In  such  cases  as  I  have  just  mentioned 
of  transmitted  immunity,  the  immunity  does  not  last  beyond 
a  single  generation.  It  has  not  become  hereditary,  it  has 
simply  been  passively  received  by  the  embryo. 

On  the  whole,  we  must  conclude  that  disease  transmission 
furnishes  no  evidence  in  favor  of  the  transmission  of  acquired 
characters.  The  most  debatable  case  is  that  of  acquired 
disease  transmitted  in  the  germ-cell.  For  practical  purposes 
this  is  heredity.  For  truly  hereditary  characters  are  often  as 
detachable  and  separate  from  the  germ-cell  as  foreign  bodies, 
as  we  shall  see  when  we  come  to  study  Mendelian  inheritance. 

3.  Induced  epilepsy.  A  famous  case  cited  in  all  discussions 
of  this  subject  is  the  case  of  Brown-Sequard's  guinea-pigs. 


I 


ACQUIRED  CHARACTERS  31 

From  1869  to  1891  Brown-Sequard  experimented  on  thou- 
sands of  guinea-pigs,  developing  methods  by  which  a  certain 
form  of  epilepsy  could  be  induced  through  injury  to  different 
parts  of  the  nervous  system,  such  as  the  spinal  cord  or  the 
sciatic  nerve.  In  some  cases  the  young  of  animals  thus 
rendered  epileptic  were  themselves  similarly  affected.  Some 
persons  who  have  repeated  Brown-Sequard's  experiments 
confirm  his  results,  notably  Romanes;  others  have  failed  to 
confirm  them. 

Weismann  has  suggested  that  some  pathogenic  organism 
may  have  got  into  the  wounds  and,  migrating  into  the  central 
nervous  system,  have  caused  the  epilepsy,  and  this  same  or- 
ganism may  have  infected  the  young.  There  is  no  evidence 
that  such  was  the  case,  however. 

Guinea-pigs  are  said  to  be  strongly  predisposed  to  epilepsy, 
and  so  the  results  of  Brown-Sequard's  experiments  may  be 
pure  coincidences,  or  due  to  the  transmission  of  a  chemical 
substance.  In  some  cases  reported  by  Brown-Sequard  the 
animals  gnawed  off  one  or  more  toes  after  the  sciatic  nerve 
had  been  cut.  Certain  of  their  young  are  reported  to  have 
done  the  same.  This  is  almost  certainly  pure  coincidence, 
since  the  evidence  as  regards  the  inheritance  of  mutilations 
is  unmistakable. 

4.  Acclimatization.  It  is  well  known  that  animals  or  plants 
taken  from  one  climate  to  another  undergo  changes  of  form. 
The  same  plant  divided  into  two  parts  and  planted  one  part 
upon  an  exposed  mountain  side,  the  other  in  a  sheltered, 
fertile  valley,  assumes  forms  very  different  in  the  two  places. 
The  mountain  form  is  short,  compact  and  dwarfed;  the  val- 
ley form  is  tall,  spreading  and  luxuriant.  It  is  assumed  by 
Lamarckians  that  these  direct  effects  of  the  environment  are 
to  some  extent  inherited,  that  if  they  are  repeated  through  a 
long  series  of  generations  they  at  last  become  habitual ^  so  to 
speak,  and  appear  spontaneously  even  when  the  external 
cause  is  lacking.  In  this  way  it  is  explained  why  mountain 
species  in  general  are  dwarfed,  and  lowland  species  are  tall 
and  luxuriant,  even  when  the  two  are  grown  side  by  side 


S2  GENETICS  AND  EUGENICS 

under  identical  conditions.  Lamar ckians  assume  that  the 
direct  effects  of  the  environment  have  accumulated  and  be- 
come hereditary.  Selectionists,  on  the  other  hand,  maintain 
that  dwarf  species  were  dwarfs  originally  and  by  nature,  and 
that  they  have  found  their  way  to  the  mountains  because 
they  alone  can  survive  under  the  harsh  conditions  there  ob- 
taining, whereas  the  more  luxuriant  forms  were  better 
adapted  to  lowland  conditions  and  have  there  crowded  out 
the  dwarfs.  It  is  evident  that  both  explanations  are  logically 
sound,  though  both  cannot  be  true.  Many  experiments  have 
been  tried  to  determine  which  best  accords  with  fact,  but  the 
results  are  not  entirely  conclusive  because  they  are  usually 
capable  of  alternative  interpretations,  and  each  one  inter- 
prets them  in  accordance  with  the  general  theory  which  he 
favors.    A  few  typical  experiments  may  be  enumerated. 

(a)  To  altered  salinity.  Paul  Bert,  many  years  ago,  at- 
tempted to  acclimatize  some  Daphniae  (small  fresh-water 
Crustacea)  to  salt  water  by  gradually  adding  salt  to  the 
aquarium.  At  the  end  of  forty-five  days,  when  the  water 
contained  1.5  per  cent  of  salt  all  the  adults  had  died;  but  the 
eggs  in  their  brood-chambers  survived,  and  the  new  genera- 
tion arising  from  these  flourished  well  in  the  salt  medium. 
This  case  has  been  cited  as  a  case  of  inherited  modification, 
but  such  it  clearly  is  not,  because  the  parents  did  not  succeed 
in  becoming  acclimatized ;  they  died  without  becoming  modi- 
fied sufficiently  to  exist  in  the  salt  water.  But  their  egg-cells 
did  become  so  modified,  and  the  animals  developing  out  of 
them  were  acclimatized,  through  direct  response  to  the  en- 
vironment, not  through  inheritance. 

Ferroniere  transferred  a  worm  {Tuhifex)  from  fresh  water 
into  sea  water.  The  animal  lived  there  and  underwent  cer- 
tain changes  of  form  (loss  of  bristles,  etc.),  which  became 
more  deeply  marked  in  later  generations.  After  several 
generations  the  animals  were  unable  to  live  in  the  original 
medium.  This  case  is  cited  as  showing  inheritance  of  an 
acquired  modification.  But  it  can  with  equal  propriety  be 
interpreted  as  showing  power  of  direct  adaptation  to  changed 


ACQUIRED  CHARACTERS  33 

environment.  It  is  doubtul  whether  any  inheritance  oc- 
curred at  all,  for  these  animals  usually  reproduce  by  fission 
and  Ferroniere's  "  several  generations  "  probably  represent 
merely  regenerated  fragments  of  one  and  the  same  original 
individual.  Had  the  transfer  back  to  fresh  water  been  gradual 
enough  there  can  be  little  doubt  that  it  would  have  been 
accomplished  successfully. 

(6)  To  a  shorter  season.  Corn  or  other  grain  taken  from  a 
southern  to  a  northern  latitude  adapts  itself  to  a  shorter 
growing  season,  maturing  earlier.  The  change  is  not  imme- 
diate, but  progressive,  the  period  required  for  maturity  grow- 
ing shorter  through  several  generations.  This  at  first  sight 
looks  like  a  good  Lamarckian  effect,  but  selectionists  regard 
it  as  equally  good  evidence  in  support  of  their  view.  For  it 
is  evident  that  the  shorter  growing  season  in  northern  lati- 
tudes would  act  as  a  selecting  agency,  killing  off  all  variations 
requiring  a  long  growing  season,  so  that  earlier  maturity 
would  become  a  racial  character. 

5.  Effects  of  changed  food  supply.  Kellogg  and  Bell  (1903) 
fed  larvae  of  the  silkmoth  on  a  reduced  quantity  of  mul- 
berry leaves  or  on  a  diet  partly  of  lettuce,  partly  of  mulberry 
leaves.  A  decrease  in  size  of  the  adult  moths  resulted  which 
persisted  through  two  subsequent  generations,  even  when 
normally  fed.  In  this  way  a  race  of  dwarf  moths  was  pro- 
duced which  however  died  out  at  the  end  of  three  genera- 
tions. This  is  not  a  clear  case  of  inherited  modification,  but 
of  direct  weakening  of  the  organism  through  mal-nutrition  or 
disease,  the  cause  whatever  it  was  being  probably  transmitted 
in  the  egg  like  "pebrine." 

Similar  but  more  extensive  experiments  were  performed 
by  Pictet  (1910-1911)  upon  larvae  of  the  gipsy-moth.  These 
larvae  feed  by  preference  on  oak  leaves.  Pictet  fed  some  on 
walnut  leaves  and  thus  obtained  moths  of  modified,  paler 
coloration.  These  modifications  became  accentuated  after 
several  generations  had  been  reared  on  walnut  leaves.  In 
one  experiment  the  modified  coloration  persisted  in  spite  of 
a  return  to  normal  diet.     The  first  generation  was  fed  on 


34  GENETICS  AND  EUGENICS 

walnut  leaves  and  presented  the  paler  coloration;  the  second 
and  third  generations  were  fed  on  oak  leaves  but  retained  the 
modified  coloration.  In  the  third  generation,  however,  the 
female  showed  partial  return  to  normal  coloration. 

Pictet  observed  some  cases  in  which  moths  became  so 
completely  accustomed  to  the  diet  of  walnut  leaves  that  their 
coloration  became  normal.  Delage  regards  this  as  greatly- 
weakening  the  case  for  inherited  modification.  He  interprets 
the  case  thus.  Walnut  leaves  are  in  general  a  poor  diet  for 
gipsy-moth  larvae.  They  weaken  the  animal.  This  weakness 
persists  through  one  or  more  generations,  doubtless  because 
of  impaired  constitution  of  the  egg,  but  is  not  certainly  trans- 
mitted as  an  acquired  character.  Indeed  the  race  may  re- 
cover from  the  weakening  produced  by  the  changed  diet. 

6.  Temperature  experiments.  Many  experiments  have  been 
performed  with  moths  and  butterflies  in  which  the  pupae 
were  subjected  to  abnormally  low  or  abnormally  high  tem- 
peratures. The  effects  of  both  extremes  are  in  many  cases 
similar.  In  general  extremely  low  or  extremely  high  tem- 
peratures produce  darker  adults.  Fischer  reared  adults  from 
pupae  of  Arctia  caja  exposed  to  a  very  low  temperature,  8°  C. 
Abnormally  dark  adults  were  obtained  in  this  way.  Some  of 
the  darkest  of  these,  produced  under  normal  conditions  un- 
usually dark  offspring.  Fischer  considers  that  the  induced 
modifications  were  transmitted.  But  this  is  far  from  certain 
for  (1)  the  moths  vary  in  darkness  of  coloration  under  normal 
conditions.  It  is  not  established  that  the  supposedly  induced 
variations  lie  outside  the  range  of  normal  variation.  (2) 
Fischer's  treatment  served  to  show  what  animals  were  nat- 
urally inclined  to  become  dark,  for  these  under  treatment 
would  become  darkest,  and  from  such  Fischer  bred.  The 
supposed  transmission  of  an  acquired  characteristic  may  be 
regarded  in  this  case  as  nothing  but  the  transmission  of  a 
natural  or  inborn  characteristic,  the  treatment  serving  as  a 
guide  to  selection. 

Weismann,  however,  influenced  by  studies  of  his  own  upon 
variation  in  color  of  butterflies  in  northern  and  in  southern 


ACQUIRED  CHARACTERS  35 

Europe,  is  willing  to  accept  at  full  face  value  such  cases  as 
this  brought  forward  by  Fischer,  and  to  allow  that  the  race 
may  become  darker  through  long-continued  subjection  to 
lower  temperatures.  He  supposes  not  that  the  body  effects 
are  transferred  to  the  germ-cells,  but  that  the  low  tempera- 
tures act  simultaneously  on  the  body  and  on  the  germ-cells, 
producing  in  them  similar  changes,  the  changes  in  the  germ- 
plasm  affecting  the  hereditary  character  of  the  race  per- 
manently. This  view  under  the  name  of  ^parallel-induction 
now  has  many  adherents.  It  is  a  practical  admission  for  a 
particular  case  of  the  Lamarckian  principle  of  evolution 
guided  in  its  course  by  environmental  action.  Whether, 
however,  Weismann  is  right  in  his  interpretation  may  still 
be  regarded  as  an  open  question. 

In  this  country,  W.  L.  Tower  (1896)  has  carried  on  exten- 
sive experiments  upon  potato  beetles  and  related  insects,  in 
which  variations  in  temperature  and  humidity  of  the  environ- 
ment have  been  followed  by  variations  in  pigmentation  of  the 
insects,  similar  to  those  observed  by  Fischer  in  the  case  of 
butterflies.  Tower  interprets  his  observations,  as  would  Weis- 
mann, as  showing,  not  inheritance  of  acquired  characters  but 
direct  modification  of  the  germ-cells,  independently  of  the 
soma.  For,  he  claims  to  have  obtained  modification  of  the 
germ-plasm,  which  accordingly  resulted  in  inherited  varia- 
tions, where  no  parallel  modification  of  the  body  of  the  parent 
had  occurred.  Inheritance  of  an  acquired  character  is  accord- 
ingly excluded  because  no  modification  was  acquired.  His 
strongest  evidence  for  this  claim  consists  of  cases  in  which 
the  same  parents  were  subjected  to  periods  of  heat  or  cold, 
alternating  with  periods  of  normal  temperature,  each  being  of 
several  weeks'  duration.  It  was  found  that  when  a  batch  of 
eggs  was  produced  in  or  immediately  following  a  period  of 
heat,  characteristic  color  variations  were  likely  to  occur 
among  the  offspring  which  may  be  called  heat  variations  and 
these  proved  hereditary.  But  when  eggs  were  produced  by 
these  same  parents  at  normal  temperatures,  no  such  varia- 
tions occurred.    Similar  effects  were  obtained  in  cold  periods. 


36  GENETICS  AND  EUGENICS 

as  contrasted  with  normal  temperatures.  While  the  bodies 
of  the  parents  remained  unaffected,  the  coloration  of  their 
offspring  varied  with  conditions  of  temperature  and  moisture 
during  the  growth  and  fertilization  of  the  eggs  which  pro- 
duced those  offspring.  Tower  therefore  concludes  that  the 
germ-plasm  was  directly  and  permanently  affected  by  varia- 
tions in  the  environment  during  a  particular  sensitive  growth 
period  of  the  egg.  This  work  is  therefore  no  argument  for  the 
inheritance  of  acquired  characters;  nevertheless  it  is  an  argu- 
ment for  evolution  directly  guided  by  the  environment,  which 
after  all  is  the  essence  of  Lamarckism.  There  are  several 
reasons  why  we  should  accept  Tower's  conclusions  with  some 
reservation. 

1.  In  the  first  place  his  experiments  are  not  reported  in 
sufficient  detail  to  enable  us  to  form  a  critical  opinion  as  to 
their  conclusiveness. 

2.  If  the  supposed  temperature  and  moisture  effects  are 
due  solely  to  those  conditions,  they  should  appear  equally  in 
all  eggs  subjected  to  the  same  conditions,  but  this  is  not  the 
case.  Only  certain  individuals  are  modified.  Since  this  is  so, 
it  is  evident  that  all  the  eggs  were  not  alike  at  the  outset,  for 
some  were  more  sensitive  than  others  to  temperature  and 
moisture  changes  in  the  environment,  if  indeed  these  were 
the  agencies  which  caused  the  changes  observed.  A  good 
argument  could  therefore  be  made  for  considering  the  tem- 
perature and  moisture  changes  as  merely  selective  agencies 
exerted  on  a  collection  of  germ-cells  already  inherently  vari- 
able in  their  potentialities.  For  Tower  maintains  that  the 
variations  once  obtained  are  perfectly  stable  for  an  indefinite 
number  of  generations.  His  claim,  therefore,  is  that  by  direct 
action  of  the  environment  for  a  comparatively  brief  period  j 
permanent  changes  in  the  germ-plasm  may  be  brought  about. 

It  would  seem  that  if  the  germ-plasm  is  thus  directly  modi- 
fiable, the  action  ought  to  be  reversible.    Changes  of  environ-        I 
ment  should  unmake  species  as  readily  as  they  make  them, 
yet  such  a  result  would  scarcely  harmonize  with  Tower's 


ACQUIRED  CHARACTERS  37 

theory,  or  with  the  known  stubborn  and  persistent  nature  of 
heritable  variations,  when  once  they  have  arisen. 

Kammerer  of  Vienna  has  pubHshed  in  the  last  five  years 
the  results  of  a  long  series  of  experiments  with  salamanders 
and  lizards  designed  to  show  the  inheritance  of  acquired 
characters.  In  this  connection  we  will  consider  his  experi- 
ments with  temperature.  The  coloration  of  several  species  of 
lizard,  with  which  Kammerer  experimented,  changes  with 
changes  of  temperature.  Kammerer  kept  lizards  at  abnor- 
mally high  or  abnormally  low  temperatures,  and  found  that 
the  induced  changes  of  coloration  persisted  to  some  extent 
even  after  the  animals  were  returned  to  normal  conditions. 
Further,  while  they  were  thus  altered,  the  offspring  which 
they  produced,  inherited  in  some  degree  the  supposedly  in- 
duced changes.  The  evidence  for  this  case,  as  for  many 
similar  cases  which  might  be  cited,  is  quite  insufficient.  Un- 
doubtedly individual  differences  in  coloration  occur  among 
the  lizards  quite  independently  of  external  temperatures. 
Further  some  probably  change  more  readily  and  extensively 
than  do  others  in  consequence  of  changed  temperatures.  A 
corresponding  variation  among  the  offspring,  plus  and  minus, 
as  compared  with  their  parents,  would  then  account  for  such 
plus  variations  in  pigmentation  as  Kammerer  observed 
among  the  offspring  and  which  he  ascribes  to  inheritance  of 
changes  induced  in  the  parents. 

Sumner  (1915)  kept  white  mice,  some  in  a  cold  room,  some 
in  a  warm  room,  where  they  multiplied.  The  mice  which 
grew  up  in  the  cold  room  had  shorter  tails  and  feet  than  those 
which  grew  up  in  the  warm  room.  Animals  reared  in  each 
room  were  now  transferred  to  a  common  room  of  ordinary 
temperature  and  allowed  to  produce  offspring  there.  In  three 
out  of  four  such  lots  of  offspring  studied,  the  cold -room 
parents  had  young  with  shorter  tails  and  feet,  but  in  a  fourth 
lot  these  relations  were  reversed.  It  seems  doubtful,  there- 
fore, whether  the  agreement  between  parents  and  offspring 
in  three  of  the  four  cases  studied  is  anything  but  a  coinci- 


38  GENETICS  AND  EUGENICS 

dence.  But  even  supposing  it  to  have  statistical  significance, 
it  may  be  due,  as  Sumner  suggests,  to  differences  directly 
impressed  upon  the  germ-cells  while  they  were  contained 
within  the  body  of  the  parent  and  the  parent  itself,  being 
very  young,  varied  in  body  temperature  with  the  room  in 
which  it  was  born.  If  so,  there  can  be  no  question  of  a 
transfer  of  an  effect  from  body  to  germ-cells,  but  only  of 
simultaneous  modification  of  the  two. 

7.  Pressure  effects.  It  is  well  known  that  pressure  has 
direct  effects  upon  the  parts  of  the  body.  The  skin  on  the 
soles  of  our  feet  is  thickened  where  our  weight  rests  upon  it, 
and  callouses  form  on  the  hand  when  it  is  used  at  hard  work. 
A  long  illness,  during  which  the  person  does  not  stand  upon 
his  feet  causes  the  thickenings  on  the  feet  in  part  to  disappear. 
They  are  undoubtedly  due  directly  to  pressure.  Yet  all  pre- 
vious generations  of  man  have  been  subjected  to  the  same 
action,  and  if  acquired  effects  are  inherited  this  should  be.  In 
fact,  it  is  found  that  in  the  foetus  of  man,  long  before  birth 
(from  five  months  on)  the  skin  is  thicker  on  the  sole  of  the 
feet  than  on  the  back  of  the  foot.  If  this  is  not  to  be  regarded 
as  an  inherited  effect  of  use  (pressure),  it  will  be  necessary  to 
explain  how  the  skin  came  to  be  thickened  originally  in  those 
particular  regions  where  use  induces  thickening. 

The  camel's  hump  has  been  cited  as  a  character  acquired 
by  pressure,  carrying  loads  on  its  back.  But  this  is  a  less 
fortunate  example  for  the  Lamarckians,  for  the  camel's  hump 
is  not  due  probably  to  pressure  at  all.  It  represents  rather  a 
reserve  food  organ,  like  special  accumulations  of  fat  in  most 
animals.  For  not  all  animals  which  carry  loads  on  their  backs 
acquire  humps,  for  example  the  ass,  the  horse.  Further, 
animals  may  acquire  humps  without  carrying  loads,  as  the 
American  bison  and  the  humped  cattle  of  India. 

8.  Light  effects.  Kammerer  has  experimented  with  the 
European  spotted  salamander  ("  fire  salamander  ")  which  is 
mottled  with  black  and  yellow  areas.  He  finds  that  if  sala- 
manders are  kept  on  a  yellow  background,  the  yellow  areas 
become  more  extensive,  while  if  the  animals  are  kept  on  a 


ACQUIRED  CHARACTERS  39 

black  background,  their  black  areas  become  more  extensive. 
Thus  there  is  an  automatic  control  of  the  color-pattern 
adapted  for  concealment,  such  as  is  known  to  occur  in  many 
fishes.  Now  Kammerer  bred  from  animals,  thus  rendered 
extremely  yellow,  and  reared  part  of  the  young  on  a  yellow 
background,  part  of  them  on  a  black  background.  Both  lots 
developed  yellow  spots  but  these  were  more  extensive  in  those 
animals  kept  on  a  yellow  background.  In  some  of  them  the 
yellow  was  more  extensive  than  in  the  parents.  This  result 
Kammerer  ascribes  to  inheritance  of  the  acquired  yellow 
coloration  added  to  the  direct  effect  of  the  yellow  background 
on  the  young.  This  conclusion  is  a  fallacious  one.  Spotted 
animals  are  extremely  variable  in  pattern,  even  when  the 
environment  does  not  change.  If  a  particular  kind  or  degree 
of  spotting  is  selected  in  the  parent  animals,  it  may  be  ex- 
pected that  offspring  will  be  obtained  both  darker  and  lighter 
than  the  parents.  In  this  way  the  race  can  by  selection  be 
made  either  darker  or  lighter,  quite  irrespective  of  any  change 
in  the  environment.  Kammerer  has  obtained  nothing  be- 
yond such  effects  as  these.  There  is  no  reason  to  think  that 
a  change  of  illumination  induced  them  to  any  greater  extent 
in  the  second  generation  than  it  did  in  the  first. 

Another  light  experiment  carried  out  by  Kammerer  seems 
to  me  to  have  more  weight.  This  was  concerned  with  the 
degeneration  of  the  eyes  in  cave  animals.  It  is  a  well-known 
fact  that  cave  animals  have  bodies  nearly  or  quite  colorless 
and  possess  degenerate  eyes.  In  animals  pigment  formation 
is  an  oxidation  process,  which  frequently  does  not  take  place 
in  the  absence  of  light.  Therefore  many  animals  which  de- 
velop in  complete  darkness  are  unpigmented.  The  human 
skin,  to  be  sure,  develops  pigment  even  in  darkness,  but  it 
develops  much  more  of  it  in  direct  sunlight.  The  skin  of  a 
European  is  fair  if  he  stays  indoors,  but  darkens  quickly  if  he 
spends  much  time  outdoors  in  the  direct  sunlight.  The  dark- 
est races  of  mankind  are  those  which  live  where  the  sunlight 
is  strongest  and  the  skies  are  clear;  the  fairest  races  live 
where  the  sun's  rays  are  less  intense  and  the  skies  are  often 


40  GENETICS  AND  EUGENICS 

overcast.  This  signifies  to  the  Lamarckian  that  the  effects  of 
the  sun's  rays  on  the  human  skin  are  inherited;  but  to  the 
selectionist  it  means  only  that  men  vary  in  depth  of  pig- 
mentation and  that  each  race  has  migrated  to  that  climate 
which  it  is  best  fitted  to  endure. 

As  regards  the  origin  of  cave  animals  the  same  diversity  of 
opinion  exists.  Some  consider  that  animals  which  found  their 
way  into  caves  lost  their  pigmentation  and  transmitted  this 
condition  to  their  offspring;  others  hold  that  such  animals 
as  were  able  to  survive  when  by  chance  they  made  their  way 
into  caves  were  probably  animals  with  little  pigmentation, 
which  could  not  very  well  exist  elsewhere. 

As  regards  the  vision  of  cave  animals,  the  Lamarckians 
hold  that  the  eyes  have  degenerated  because  no  longer  used, 
whereas  the  selectionists  hold  that  the  animals  which  have 
taken  to  living  in  caves  have  been  driven  to  this  course  by 
the  degeneration  of  their  eyes,  and  they  point  out  that  the 
nearest  relatives  of  cave  animals  are  those  with  poorly 
developed  eyes,  which  live  in  semi-darkness. 

Kammerer,  very  commendably,  has  put  these  alternative 
views  to  an  experimental  test.  He  has  reared  in  daylight  the 
young  of  the  cave  salamander,  Proteus  anguinius.  Under 
these  circumstances  the  skin  became  pigmented  and  the  eye 
did  not  degenerate,  as  normally;  but  if  the  animals  were  kept 
in  strong  light  continuously  the  skin  became  so  heavily  pig- 
mented, including  that  in  front  of  the  eye  where  the  trans- 
parent cornea  forms  in  ordinary  animals  living  in  the  light, 
that  in  consequence  the  eye  itself  degenerated.  To  overcome 
this  difficulty  Kammerer  kept  the  animals  in  red  light,  which 
is  less  favorable  than  daylight  to  pigment  formation,  but 
suffices  nevertheless  to  stimulate  the  eyes  to  development. 
The  red-light  treatment  was  given  for  one  week  out  of  three 
during  the  first  eighteen  months  of  the  animals'  lives.  In 
this  way  the  eye,  which  in  cave-inhabiting  individuals  is  very 
small  and  rudimentary,  was  brought  to  full  development, 
with  a  transparent  cornea  and  all  other  parts  necessary  for 
vision. 


ACQUIRED  CHARACTERS  41 

This  result  leaves  no  doubt  that  light  is  a  necessary  stimu- 
lus for  full  development  of  the  eye  in  Proteus,  and  it  is  the 
absence  of  this  stimulus  which  has  led  in  part  to  the  present 
degenerate  condition  of  the  eye.  Whether  or  not  the  degen- 
eration has  advanced  from  generation  to  generation  is  of 
course  conjectural,  but  seems  highly  probable.  Weismann 
indeed  considered  the  evidence  for  the  progressive  degenera- 
tion of  disused  organs  so  strong  that  he  framed  a  special 
hypothesis,  that  of  germinal  selection,  to  account  for  it.  To 
this  matter  we  shall  return  later. 

9.  Instincts,  Instincts  are  among  the  most  vital  posses- 
sions of  animals,  but  the  same  difference  of  opinion  exists  as 
regards  their  origin  as  concerning  the  origin  of  other  adap- 
tive characteristics  of  organisms.  Without  being  taught, 
animals  do  generation  after  generation  the  same  acts  in  the 
same  way.  They  seem  to  know,  without  individual  experi- 
ence or  education,  exactly  what  to  eat,  and  how  to  secure  it; 
how  to  prepare  a  nest  or  burrow  of  a  very  definite  pattern; 
how  to  care  for  young,  though  they  have  never  seen  young 
cared  for  before;  what  to  do  as  the  seasons  change;  and 
numberless  other  vital  and  necessary  things.  Some  say  this 
is  inherited  memory,  nothing  less;  the  ancestors  have  learned, 
their  descendants  remember.  Just  as  brain  cells,  after  re- 
ceiving a  variety  of  sensations  one  after  another,  are  able  to 
reproduce  them  again  in  the  same  order  and  complexity 
through  memory,  so  the  reproductive  cells  become  store- 
houses of  racial  experience  or  habit  which  they  transmit  as 
instincts.  This  easy  way  of  accounting  for  instincts  as  habits 
registered  like  phonograph  records  in  the  germ-plasm  has 
even  been  extended  to  all  inJieritance  by  a  number  of  writers, 
represented  at  the  present  time  by  Richard  Semon.  This 
idea  had  great  influence  in  America  in  the  last  quarter  of  the 
last  century,  when  a  strong  school  of  modern  Lamarckians, 
or  neo-Lamarckians,  flourished  here.  Many  still  hold  to 
this  view,  but  the  neo-Darwinians,  or  followers  of  Weismann, 
have  of  late  been  rather  in  the  ascendancy.  In  their  view, 
instincts  arise  because  the  structure  of  the  germ-plasm  neces- 


42  GENETICS  AND  EUGENICS 

sitates  a  particular  response  when  certain  external  stimuli  are 
operative,  not  at  all  because  such  a  response  has  before  been 
made  by  the  ancestors.  Having  denied  that  action  of  the 
individual  can  affect  the  germ-plasm  within  it,  they  can  con- 
ceive of  no  mechanism  for  the  transmission  of  habits  formed 
by  the  individual,  and  so  deny  the  existence  of  such  trans- 
mission. 

On  the  neo-Lamarckian  view  a  hen  sits  on  eggs  because 
her  ancestors  have  formed  the  habit  of  incubating  eggs;  on 
the  Weismannian  view  the  hen  sits  on  eggs  because  she  can- 
not help  doing  it;  when  she  is  in  a  certain  physiological  state 
and  the  nest  of  eggs  is  there,  she  sits,  and  that  is  all  there  is 
to  it.  Neither  of  these  views  is  very  satisfying.  On  one 
hand  the  neo-Lamarckian  fails  to  explain  how  the  first  hen 
came  to  incubate,  which  the  Weismannian  glibly  states  is 
just  because  she  is  built  that  way;  her  germ-plasm  necessi- 
tates it.  On  the  other  hand,  the  Weismannian  can  give  us 
no  suggestion  as  to  how  structural  conditions  of  the  germ- 
plasm  can  cause  a  hen  to  sit  rather  than  to  crow,  when  a  nest 
of  eggs  is  before  her,  but  the  well-established  effects  of 
internal  secretions  come  here  to  his  rescue. 

The  whole  question  of  the  relation  of  instincts  to  inheri- 
tance is  very  perplexing.  At  present  we  can  make  very  little 
out  of  it,  yet  there  can  be  no  doubt  that  it  concerns  vitally 
our  fundamental  theories  of  evolution  and  such  applied  fields 
as  Eugenics. 

The  correct  attitude  in  the  study  of  instincts  is  maintained 
by  those  who  are  seeking  to  learn  how  much  each  instinct 
involves,  and  to  what  extent  imitation  and  education  supple- 
ment or  modify  it.  So  far  as  possible  each  instinct  should  be 
resolved  into  terms  of  response  to  external  chemical  or  physi- 
cal changes,  or  to  internal  physiological  states.  For  example 
it  was  observed  many  years  ago  that  certain  small  Crustacea 
instinctively  swim  toward  a  light.  More  careful  study  showed 
that  they  do  so  only  under  particular  conditions.  If  the 
temperature  of  the  water  is  raised,  or  its  salinity  increased, 
the  animal  may  reverse  its  response  and  swim  away  from  the 


ACQUIRED  CHARACTERS  43 

source  of  light.  These  are  changes  of  external  conditions 
which  modify  the  instinctive  response.  Internal  or  physio- 
logical states  of  the  animal  may  also  modify  the  instinctive 
responses.  Thus,  if  the  crustacean  has  been  subjected  to 
mechanical  stimulation  (repeated  touching  with  a  sohd  ob- 
ject) its  response  may  be  altered. 

Again  larvae  of  a  barnacle  for  a  few  minutes  after  hatching 
swim  toward  the  light,  then  they  turn  and  swim  away  from  it, 
a  series  of  responses  calculated  to  bring  them  to  suitable 
spots  for  attachment.  The  response  has  been  modified 
through  some  internal  physiological  change.  Larvae  of  the 
brown-tail  moth,  after  their  winter  fast,  are  strongly  positively 
phototropic.  They  migrate  up  to  the  tips  of  the  branches  to 
feed  on  the  opening  buds.  If  at  this  time  they  are  brought 
into  the  laboratory  and  placed  in  a  test  tube,  they  go  toward 
the  window  and  will  remain  at  the  end  of  the  tube  toward  the 
window  until  they  die,  even  if  food  is  at  the  opposite  end  of 
the  tube  a  few  inches  away.  After  the  larvae  have  fed  they 
are  no  longer  phototropic.  Digestion  has  probably  destroyed 
the  substance  in  their  bodies  on  which  their  phototropism 
depended.    (Loeb,  Yale  Review,  July,  1915.) 

By  such  methods  of  studying  the  instincts  of  animals  the 
problem  of  instinct  formation  and  inheritance  may  be  simpli- 
fied, through  the  elimination  from  it  of  all  non-essential  and 
outside  elements. 

As  intelligence  increases  in  the  animal  kingdom,  we  find 
that  instinct  sinks  more  and  more  into  a  subordinate  position. 
In  man  there  is  very  little  inherited  knowledge,  if  instinct 
may  so  be  regarded ;  nearly  everything  has  to  be  learned  from 
the  beginning.  Nevertheless  it  is  an  open  question  whether 
intelligence  has  not  increased  through  use,  whether  we  do  not 
learn  more  easily  for  the  reason  that  our  ancestors  have  for  a 
million  generations  been  learners.  Of  course  I  do  not  refer 
here  to  formal  education,  but  only  to  the  exercise  of  such 
intelligence  as  distinguishes  man  from  other  animals.  May 
not  this  have  been  evolved  in  part  through  use  ? 


44  GENETICS  AND  EUGENICS 

Summary.  Notwithstanding  the  fundamental  nature  of 
the  problem  of  the  inheritance  of  acquired  characters,  and  all 
that  has  been  said  and  done  to  solve  it,  it  still  remains  an 
unsolved  problem.  So  far  as  the  inheritance  of  mutilations, 
disease,  and  induced  epilepsy  are  concerned,  the  evidence  is 
negative  or  inconclusive.  Acclimatization,  the  effects  of 
changed  food  supply,  and  temperature  effects  can  be  ex- 
plained quite  as  well  on  other  grounds  as  on  that  of  the  in- 
heritance of  acquired  characters.  Pressure  and  light  effects 
are  somewhat  more  easily  explained  as  cumulative  from 
generation  to  generation,  i.  e.,  as  inherited  acquired  charac- 
ters, than  as  due  merely  to  germinal  variation.  The  same  is 
true  of  instincts,  which,  if  interpreted  as  inherited  habits, 
afford  the  strongest  outstanding  evidence  for  the  inheritance 
of  acquired  characters.  Nevertheless  even  here  an  alterna- 
tive explanation  is  possible. 

The  Lamarckian  view  has  been  shown  by  the  critical  work 
of  Weismann  and  his  followers  to  be  inapplicable  to  many 
groups  of  cases  to  which  it  had  previously  been  applied.  This 
is  a  real  service  on  the  part  of  Weismann.  Nevertheless,  in 
fields  where  the  Lamarckian  principle  has  not  yet  been  dis- 
proved, viz.,  as  regards  the  effects  of  use  and  disuse,  it 
affords  an  easier  and  fuller  explanation  of  progressive  evolu- 
tion and  of  adaptation  in  particular  than  does  the  selectionist 
view.  Further,  Weismann  and  his  followers  have  been  forced 
practically  to  concede  the  existence  of  Lamarckian  evolution, 
that  is  evolution  the  course  of  which  is  guided  in  adaptive 
directions  by  the  environment.  For  Weismann  admits  that 
the  environment  may  cause  'parallel  modifications  of  soma 
and  germ-plasm.  For  practical  purposes  this  is  just  as  effec- 
tive in  guiding  evolution  as  if  the  soma  first  developed  modifi- 
cations and  then  handed  them  on  to  the  germ-cells.  That  a 
mechanism  for  the  transmission  of  acquired  characters  from 
soma  to  germ-cells  has  as  yet  not  been  demonstrated,  does  not 
of  course  disprove  the  existence  of  such  a  mechanism.  Such 
phenomena  as  memory,  having  its  basis  in  the  nervous  sys- 
tem, and  as  the  control  of  development  and  of  behavior 


ACQUIRED  CHARACTERS  45 

through  internal  secretions,  give  us  grounds  for  believing  that 
an  adequate  basis  will  be  found  when  our  knowledge  of  the 
organism  becomes  more  complete. 

The  problem  of  acquired  characters,  after  all,  concerns  only 
the  higher  animals.  In  the  lower  animals  and  in  plants  no 
such  sharp  distinction  exists  between  body  and  germ-cells 
as  we  find  in  the  higher  animals.  We  may  reproduce  the 
entire  plant  from  a  cutting  of  root,  stem,  or  even  a  leaf  in 
some  cases.  Hence  there  is  more  chance  in  such  cases  of 
direct  modification  of  the  cells  capable  of  reproduction,  for 
most  of  the  cells  of  the  plant  retain  this  capacity.  In  the 
lowest  organisms  {protozoa,  bacteria)  there  is  no  distinction 
whatever  between  body  and  germ-cells.  Every  cell  is  capable 
of  reproduction;  and  modifications  produced  in  a  cell  by  the 
environment  are  handed  on  directly  to  the  next  generation. 
For  example  medical  men  have  learned  how  to  decrease  the 
virulence  of  diseases  at  will  by  heat  or  chemicals  acting 
directly  on  the  disease  germs.  They  are  thus  able  to  confer 
immunity  to  a  virulent  disease  by  first  producing  and 
then  introducing  into  the  body  a  feeble  form  of  the  same 
disease. 

If  in  the  lower  organisms  the  potentialities  of  living  sub- 
stance can  thus  be  altered,  it  seems  reasonable  to  suppose 
that  the  same  possibility  may  exist  in  the  higher  animals  and 
plants,  provided  agencies  capable  of  producing  change  are 
allowed  to  act  on  the  germinal  substance.  It  is  the  sheltered 
position  of  the  germ-cells  which  seems  ordinarily  to  exempt 
them  from  direct  modification,  but  we  cannot  safely  assume 
that  they  are  in  all  cases  free  from  such  modification.  Experi- 
ments of  Stockard  show  that  in  guinea-pigs  repeatedly  in- 
toxicated with  alcohol,  the  germ-cells  are  enfeebled  so  that 
offspring  of  such  parents,  whether  male  or  female,  are  more 
likely  to  be  feeble  and  sickly,  and  so  to  die.  Experiments  of 
Hertwig  show  that  similarly  the  germ-cells  of  frogs  are 
capable  of  being  injured  by  emanations  of  radium  in  conse- 
quence of  which  enfeebled  or  abnormal  offspring  may  be 
produced. 


46  GENETICS  AND  EUGENICS  i 

If  the  germ-cells  are  thus  capable  of  modification,  evolu- 
tion guided  by  the  environment  must  be  in  some  measure  at 
least  a  reality.  The  truth  then  lies  neither  in  the  extreme 
Lamarckian  view  that  all  acquired  characters  are  inherited 
nor  in  the  extreme  Weismannian  view,  that  no  extraneous 
influences  modify  the  germ-plasm,  but  somewhere  in 
between. 


CHAPTER  IV 

WEISMANN'S  THEORY  OF  HEREDITY 

Weismann  believed  that  a  new  type  of  organism  arises  only 
in  consequence  of  the  origin  of  a  new  type  of  germ-cell.  If  he 
had  been  asked  the  ancient  riddle,  "  which  was  created  first, 
the  egg  or  the  hen,"  he  would  undoubtedly  have  answered, 
"  the  egg.''  He  would  have  explained  that  the  first  bird  came 
from  a  new  type  of  egg  laid  by  a  reptile-like  ancestor. 
Changed  structure  of  the  germ-plasm  must  result,  he 
thought,  in  changed  structure  of  the  organism  developing 
from  it;  and  he  would  scarcely  have  admitted  that  a  new 
sort  of  organism  might  arise  in  any  other  way.  But  the 
experimental  study  of  the  development  of  organisms  has 
shown  that  the  germ-plasm  forms  only  one  of  two  comple- 
mentary sets  of  agencies  which  determine  what  the  adult 
organism  shall  be.  It  is  true  that  the  character  of  the  germ- 
cell  determines  in  part  what  the  character  of  the  adult  organ- 
ism shall  be,  but  so  also  does  the  environment.  If  we  plant 
beans,  we  must  expect  to  harvest  beans  not  corn,  but  whether 
the  harvest  is  large  or  small  will  depend  upon  the  soil  and  the 
season.  Sunlight,  moisture,  a  suitable  temperature,  and 
proper  chemical  substances  in  the  soil  are  all  indispensable 
conditions  to  the  production  of  any  crop  at  all,  and  they  con- 
trol within  hmits  the  size,  vigor,  and  productiveness  of  the 
plants  grown.  Both  internal  and  external  agencies  influence 
the  form  of  organisms.  These  are  sunmiarized  in  the  two 
words,  heredity  and  environment.  Weismann  emphasized 
the  first  almost  to  the  neglect  of  the  second.  Lamarck  had 
previously  gone  to  the  opposite  extreme,  emphasizing  the  im- 
portance of  the  environment  not  only  in  directly  adapting  the 
organism  to   its  surroundings  but  also  in  controlling  its 

47 


48  GENETICS  AND  EUGENICS 

heredity.    It  is  coming  to  be  recognized  that  the  truth  hes 
somewhere  between  these  extreme  views. 

WTiat  in  general  were  Weismann's  views  and  how  did  he 
arrive  at  them  ? 

Weismann's  Method 

Weismann's  method  of  constructing  an  hypothesis  to 
account  for  heredity  differed  fundamentally  from  Darwin's. 
Darwin  reasoned  inductively,  Weismann  deductively.  Dar- 
win tried  first  to  ascertain  what  characteristics  are  inherited 
and  then  to  imagine  a  mechanism  which  might  explain  their 
inheritance.  The  result  was  "  pangenesis."  Weismann,  on 
the  other  hand,  first  inquired  what  is  the  mechanism  of  in- 
heritance and,  having  answered  this  to  his  own  satisfaction, 
proceeded  to  the  conclusion  that  only  such  characters  are 
inherited  as  have  their  basis  in  this  mechanism.  The  result 
was  the  chromosome  theory  of  inheritance.  It  has  this  fea- 
ture in  common  with  "  pangenesis,"  the  inherited  character- 
istics are  supposed  to  be  determined  in  advance  and  to  be 
represented  in  the  germ-cell  by  material  bodies.  These  are 
the  "  gemmules  "  of  Darwin,  the  "  determiners  "  of  Weis- 
mann. Darwin  supposed  that  the  "  gemmules  "  migrate 
from  all  parts  of  the  body  into  the  germ-cells  and  so  make  it 
inevitable  that  the  organism  which  develops  out  of  the  germ- 
cell  shall  have  the  same  parts  and  properties  as  the  parent. 
As  regards  the  origin  of  variations,  pangenesis  might  be 
called  a  centripetal  theory,  since  determiners  are  supposed 
by  it  to  migrate  centrally  tow^ard  the  germ-cells. 

Weismann's  theory,  on  the  other  hand,  is  centrifugal;  he 
supposes  that  the  "  determiners  "  originate  solely  in  the 
germ-plasm  and  migrate  thence  out  into  the  various  parts  of 
the  developing  body  and  that  thus  differentiation  is  pro- 
duced. There  is  on  his  view  no  centripetal  movement  of 
determiners  whatever;  they  never  pass  from  soma  to  germ- 
cells,  but  only  in  the  reverse  direction. 


WEISMANN'S  THEORY  OF  HEREDITY  49 

Weismann's  Mechanism  of  Heredity 

Weismann  had  this  advantage  over  Darwin;  in  his  time 
knowledge  of  the  structure  of  the  germ-cells  had  considerably 
increased  over  what  it  was  when  Darwin  conceived  the  hy- 
pothesis of  pangenesis. 

Weismann  identified  his  "  determiners  "  with  certain  con- 
spicuous structures  of  the  germ-cell  called  chromosomes 
(unknown  in  Darwin's  time),  and  supposed  that  the  nature 
of  these  determines  and  controls  the  nature  and  activity  of 
the  cell  containing  them. 

It  is  the  theoretical  importance  which  Weismann  and 
others  have  assigned  to  these  structures  that  has  given  them 
their  great  prominence  in  the  study  and  description  of  cell 
phenomena  in  the  last  thirty  years.  In  reality  the  chromo- 
somes make  up  a  part  only  of  the  germ-cell  and  we  have  no 
certain  knowledge  that  they  form  the  more  important  part. 
Nevertheless  a  majority  of  biologists,  probably,  at  the  present 
time  believe  with  Weismann  that  heredity  is  due  to  material 
substances  or  determiners  which  are  located  in  the  chromo- 
somes.   The  principal  reasons  for  so  thinking  are: 

1.  The  conspicuousness  of  the  chromosomes  at  the  tune 
of  cell  division  and  the  very  exact  manner  in  which  as  a  rule 
each  of  them  divides  into  two  equal  parts,  which  pass  into 
different  cell-products. 

2.  The  constancy  of  the  number  of  the  chromosomes  in 
the  same  species  of  animal  or  plant.  The  number  is  different 
in  different  species  but  within  the  same  species  it  is  very  con- 
stant. The  only  known  exceptions  to  this  rule  are  such  as 
may  be  cited  in  support  of  the  general  idea  that  chromosomes 
are  determiners  of  heredity. 

(a)  The  two  sexes  within  the  same  species  frequently  differ 
as  regards  the  number  of  chromosomes  in  their  genn-cells. 
When  this  is  the  case  the  male  has  the  smaller  number  of 
chromosomes,  and  it  is  assumed  that  the  chromosome  or 
chromosomes  which  the  male  lacks  determine  femaleness. 

(6)  It  has  been  shown  in  the  case  of  the  evening  primroses 
(Oenothera)  that  a  particular  heritable  type  of  variation 


50  GENETICS  AND  EUGENICS 

("  lata  mutant  ")  contains  one  more  chromosome  than  the 
parent  species  from  which  it  has  been  observed  repeatedly  to 
arise.  Another  type  of  mutant  in  this  same  group  of  plants 
contains  twice  the  ordinary  number  of  chromosomes  ("  gig  as 
mutant,"  Gates,  1915).  The  fact  that  visible  characters  of 
the  organism  vary  simultaneously  with  variation  in  the 
chromosomes  creates  a  presumption  that  the  relationship  is 
a  causal  one. 

3.  The  experimental  evidence  shows  that  in  general  the 
father  is  just  as  influential  as  the  mother  in  determining  the 
inheritance  of  the  children.  But  the  egg-cell  is  vastly  larger 
than  the  sperm-cell.  Therefore  much  of  the  substance  of  the 
egg  cannot  be  concerned  in  heredity.  What  the  egg  and 
sperm-cell  have  in  common  consists  more  largely  of  chromatin 
than  of  any  other  substance.  This  makes  it  seem  probable  that 
chromatin  is  concerned  in  heredity. 

4.  There  exists  a  parallelism  between  the  behavior  of  the 
chromosomes  in  the  development  of  the  germ-cells  and  that 
of  certain  characteristics  in  heredity.  It  is  supposed,  there- 
fore, that  the  chromosomes  actually  contain  chemical  sub- 
stances necessary  for  the  development  of  these  inherited 
characters  and  in  this  sense  are  determiners  of  heredity. 

The  assumption  of  Weismann  that  heredity  is  due  to  deter- 
miners contained  in  the  germ-cell,  like  the  pangenesis  theory 
of  Darwin,  has  encountered  many  difficulties.  Consequently 
numerous  supplementary  hypotheses  have  been  found  neces- 
sary to  enable  it  to  feature  as  a  general  explanation  of  the 
facts  of  inheritance. 

Difficulties  Encountered  by  Weismann's  Theory 

1.  Development  (ontogeny).  The  first  diflSculty  en- 
countered lay  in  the  explanation  of  the  development  of  the 
individual  from  the  egg.  Weismann  assumed  that  each  cell 
owes  its  peculiar  form  and  activities  to  the  determiners  which 
it  contains,  these  being  located  in  its  chromosomes.  Since 
the  cells  composing  the  different  parts  and  tissues  of  the  body 
differ  in  their  forms  and  activities,  it  was  necessary  to  assume 


WEISMANN'S  THEORY  OF  HEREDITY  51 

further  that  the  different  kinds  of  cells  contain  different  de- 
terminers and  consequently  that  as  the  egg  divides  up  into 
cells  which  form  the  different  parts  of  the  body,  these  cells 
must  receive  different  determiners.  But  microscopic  exami- 
nation of  the  cells  of  the  body  reveals  no  such  differences;  it 
shows  differences  in  pretty  much  everything  except  chromo- 
somes, which  remain  remarkably  constant. 

Boveri  (1887)  has  described  one  case  which  seems  to 
support  the  idea  that  changes  in  the  chromatin  occur,  as 
body-cells  become  distinguishable  from  germ-cells.  In  the 
parasitic  worm,  Ascaris,  the  chromosomes  are  seen  partially 
to  break  up  and  disintegrate  in  those  cells  of  the  embryo  from 
which  the  body  arises,  whereas  the  original  ovarian  structure 
remains  unmodified  in  the  germ-cells.  No  similar  case,  how- 
ever, has  been  described  in  other  organisms,  so  that  it  seems 
very  doubtful  whether  the  observed  changes  have  the  signifi- 
cance originally  attached  to  them  by  Boveri.^  There  are  good 
reasons  for  believing  that  the  chromatin  content  of  each  cell 
of  the  body  is  like  that  of  every  other  cell  of  the  same  body, 
and  that  differentiation  results  either  (a)  from  the  position 
of  a  cell  in  relation  to  other  cells,  which  will  accordingly  regu- 
late its  intake  and  output,  or  (6)  from  an  original  difference 
in  substance  contained  in  the  cytoplasm  of  the  cell  (the  extra- 
nuclear  part).  Such  cytoplasmic  differences  between  cells 
arise,  during  development,  from  the  fact  that  the  egg  cyto- 
plasm, at  the  beginning  of  development,  is  not  homogeneous, 
and  consequently  the  cells  into  which  the  egg  divides  are  not 
alike  in  cytoplasmic  content. 

2.  Regeneration.  A  man  who  loses  a  leg  or  an  arm  is  de- 
prived of  the  same  for  the  remainder  of  his  life,  but  many  of 
the  lower  animals  can  restore  lost  parts  by  a  process  which 
we  call  regeneration.  If  a  young  salamander,  a  crab  or  a 
lobster  is  deprived  of  a  leg,  a  new  leg  grows  out  again  from 

1  It  is  true  that  Hegner  (1914),  confirming  Kahle  (1908),  has  also  observed 
"diminution  of  chromatin"  occurring  in  the  differentiation  of  somatic  cells  in  an 
insect,  Miastor,  but  in  numerous  other  animals  studied  by  Hegner  he  has  found 
no  such  diminution  of  chromatin  but  has  observed  the  germ-cells  to  be  differen- 
tiated solely  by  cytoplasmic  changes. 


52  GENETICS  AND  EUGENICS 

the  stump  of  the  old  one.  Such  facts  as  these  compelled 
Weismann  to  assume  that,  in  cases  of  leg  regeneration,  not 
all  the  leg  determiners  pass  out  during  development  into  the 
leg,  but  a  supply  is  also  held  in  reserve  in  the  adjacent  parts 
of  the  body;  these  being  latent  or  inactive  ordinarily,  but 
becoming  active  when  the  leg  is  removed. 

Experimental  studies  of  regeneration  made  by  Morgan, 
Child,  and  others  scarcely  support  Weismann's  view.  They 
indicate  that  any  undifferential  cell  of  the  body,  if  placed  at 
the  stump  of  an  amputated  leg,  might  function  in  leg  re- 
generation, and  so  that  specific  leg  regenerators  do  not  exist. 
It  is  true  that,  in  many  animals,  particular  groups  of  cells 
have  the  ability  to  produce  only  a  particular  kind  of  struc- 
ture, no  matter  where  they  are  placed  in  the  body,  in  a 
transplantation  experiment.  But  in  such  cases  it  is  pretty 
clear  that  we  are  dealing,  not  with  the  effects  of  specific  deter- 
miners, but  with  the  consequences  of  cytoplasmic  differentia- 
tion which,  in  many  cases  at  least,  arose  in  the  undivided  ^gg 
when  no  nuclear  difference  existed  within  the  organism,  since 
it  contained  only  a  single  nucleus. 

3.  Polymorphism.  In  many  species  of  animals  and  plants 
the  form  of  the  adult  differs  fundamentally  according  to  the 
environment  in  which  it  is  placed.  In  certain  amphibious 
plants  (e.  g.,  Ranunculus  aquatilis)  the  plant  when  growing 
in  the  air  develops  flat  broad  leaves,  but  when  growing  under 
water  develops  leaves  dissected  into  numerous  hairlike  ap- 
pendages. Weismann  supposed  that  in  such  cases  there  exist 
alternative  sets  of  determiners  in  the  germ-plasm,  one  for  the 
land  form  of  leaf,  one  for  the  water  form,  conditions  of  dry- 
ness or  dampness  during  development  calling  one  or  the  other 
set  into  activity.  If  intermediate  conditions  were  shown  to 
produce  intermediate  effects,  he  would  doubtless  assume  a 
joint  and  partial  activity  of  both  sets.  In  animals  more 
complicated  conditions  of  polymorphism  occur.  Many  spe- 
cies of  butterfly  have  spring  and  summer  generations  of  off- 
spring (broods  as  they  are  called),  quite  different  in  appear- 
ance, corresponding  to  different  external  conditions  of  tem- 


WEISMANN'S  THEORY  OF  HEREDITY  53 

perature  or  food  supply.  The  gall  insects  of  oak  and  willow 
trees  have  summer  and  winter  generations  very  different  in 
character.  The  summer  generation  usually  feeds  upon  the 
soft  tissues  of  the  growing  leaf  and  produces  winged  adults  of 
both  sexes;  whereas  the  winter  generation  feeding  on  the 
woody  tissues  produced  by  a  stem  or  metamorphosed  bud, 
may  consist  of  wingless  females  only,  which  lay  unfertilized, 
i.  e.,  parthenogenetic  eggs.  In  such  cases  Weismann  sup- 
poses that  alternative  sets  of  determiners  exist  in  the  germ- 
plasm,  which  are  activated  by  summer  or  by  winter  condi- 
tions respectively. 

The  case  of  the  social  insects  (bees  and  ants)  is  still  more 
complicated ;  here  there  may  exist  four  or  ^ve  different  adult 
forms  as  drones  (males)  queens  (egg-laying  females)  and 
workers  or  soldiers  of  various  sorts.  The  workers  and  soldiers 
are  all  imperfectly  developed  females,  not  producing  eggs 
ordinarily  but  merely  taking  care  of  the  rest  of  the  colony. 
Experiment  has  shown  that  the  same  egg,  in  the  case  of  the 
honeybee,  may  produce  either  a  queen  or  a  worker,  depend- 
ing upon  the  amount  and  quality  of  the  food  suppHed  to  the 
developing  larva.  The  same  is  undoubtedly  true  of  the 
various  sorts  of  soldiers,  among  other  social  insects,  these  be- 
ing alternative  forms  of  the  female.  Weismann  supposes 
that  there  are  as  many  distinct  sets  of  determiners  in  the  egg 
as  there  are  different  forms  into  which  it  may  develop.  This 
line  of  explanation  assigns  to  determiners  located  within  the 
nucleus  of  the  egg,  influences  which  demonstrably  lie  outside 
the  egg.  As  an  explanation  of  polymorphism  the  theory  of 
alternative  nuclear  determiners  is  not  only  superfluous  but 
also  positively  erroneous. 

4.  Variation.  Weismann  supposed  that  all  variations 
originate  in  the  germ-plasm,  and  subsequently  find  expres- 
sion in  the  body  of  the  offspring,  reversing  the  idea  of  La- 
marck and  Darwin,  who  supposed  that  variations  first  origi- 
nate in  the  body  and  are  thence  transferred  to  the  germ-cells. 
To  account  for  adaptive  variation,  Weismann  framed  two 
supplementary  hypotheses.    1.   To  account  for  the  origin  of 


54  GENETICS  AND  EUGENICS 

inherited  variations  similar  to  those  which  the  environment 
directly  produces  in  the  body,  he  invented  the  hypothesis  of 
parallel  modification  of  germ-plasm  and  soma,  to  which  refer- 
ence has  already  been  made.  2.  To  account  for  the  appar- 
ent inheritance  of  the  effects  of  use  and  disuse,  he  invented 
the  hypothesis  of  germinal  selection.  On  this  view  the  various 
determiners  which  compose  the  germ-plasm  are  competing 
with  each  other  in  a  struggle  for  nourishment,  just  as  animals 
and  plants  struggle  with  each  other  for  existence  in  the  world 
at  large.  Sometimes  one  determiner  gets  more  nourishment, 
sometimes  another;  but  whichever  one  gets  most  nourish- 
ment, grows  largest,  and  would  consequently  give  rise  to  a 
plus  variation  of  a  corresponding  part  or  organ  of  the  body. 
When  one  determiner  gets  more  nourishment,  that  is,  pro- 
duces a  plus  variation,  some  other  determiner  gets  less  and 
so  produces  a  minus  variation.  Thus  there  is  perpetual  varia- 
tion in  the  parts  and  organs  of  the  body,  which  affords 
abundant  material  for  natural  selection  to  act  upon.  For  if 
any  essential  organ  gets  too  small,  its  possessor  is  eliminated. 
But  if  the  organ  which  undergoes  minus  variation  is  a  use- 
less one,  no  disadvantage  results  to  the  organism;  on  the 
contrary,  there  is  more  nourishment  left  for  essential  organs, 
which  therefore  grow  at  the  expense  of  the  useless  ones. 
Thus  through  natural  selection  useless  organs  tend  to  dimin- 
ish and  ultimately  to  disappear  altogether,  while  essential 
organs  (those  most  used)  grow  in  size  and  activity.  An 
apparent  inheritance  of  the  effects  of  use  and  disuse  results. 
Modern  research  supports  Weismann's  theory  of  nuclear 
determiners  to  this  extent.  It  appears  highly  probable  that 
special  chemical  substances  necessary  for  the  production  of 
particular  variations  are  located  in  particular  parts  of  the 
cell,  possibly  in  chromosomes.  It  is  also  conceivable  that 
these  substances  may  vary  from  cell  to  cell  in  amount  or 
quality,  and  that  under  a  constant  environment  variation  in 
particular  organs  affected  may  thus  result.  But  it  is  not  neces- 
sary to  suppose,  as  Weismann  did,  that  these  groups  of  sub- 
stances are  engaged  in  a  struggle  of  any  sort,  with  each  other. 


CHAPTER  V 

ATTEMPTS  TO  CLASSIFY  AND  MEASURE  VARIATION: 

BIOMETRY 

The  period  from  1880  to  1900,  following  Darwin's  death,  was 
marked  by  extreme  speculation  concerning  evolution  rather 
than  by  inductive  study  of  its  phenomena.  This  speculative 
tendency  found  its  culmination  in  Weismann's  brilliant  es- 
says, but  his  ideas,  notwithstanding  their  brilliancy,  failed  to 
win  acceptance  among  such  biologists  as  insisted  on  having  a 
substantial  basis  of  well-ascertained  facts  on  which  to  rest 
their  theories.  Weismann's  theories  were  accordingly  dis- 
tinctly on  the  wane  when  in  1900  they  received  support  from 
an  unexpected  source,  the  rediscovery  of  Mendel's  law  of 
heredity,  which  now  fully  established  seems  to  require  for  its 
explanation  some  such  system  of  determiners  as  Weismann 
had  hypothecated  and  located  in  the  chromosomes. 

During  this  period  of  speculation  about  evolution,  biolo- 
gists had  been  looking  in  various  directions  for  new  tools 
with  which  to  attack  the  study  of  evolutionary  problems. 
The  facts  of  development  were  more  carefully  studied  and 
accurately  described  than  ever  before,  and  more  precise  in- 
formation was  sought  about  the  influence  of  environment 
upon  development  and  growth.  Thus  experimental  embry- 
ology and  experimental  morphology  were  born,  to  be  followed 
a  little  later  by  experimental  breeding.  Meantime,  Bateson 
was  attempting  to  classify  variations  on  morphological 
grounds  without  reference  to  their  causation,  and  Pearson 
was  seeking  to  measure  variability  so  as  to  determine  its 
direction  and  rate  of  progress. 

Darwin  had  throughout  nearly  a  lifetime  collected  all  ob- 
tainable facts  about  variation  in  animals  and  plants  as  a 
basis  for  his  generalizations  concerning  evolution  and  hered- 
ity.    Much  of  his  data  is  contained  in  his  work  on  the 

55 


56  GENETICS  AND  EUGENICS 

Variation  of  Animals  and  Plants  under  Domestication. 
Bateson  took  up  this  work  after  Darwin's  death  and  collected 
a  large  number  of  facts  concerning  variation,  which  he  at- 
tempted to  classify,  but  without  great  success.  His  results 
are  found  in  a  book  entitled  Materials  for  the  Study  of  Va- 
riation, published  in  1894.  The  most  important  conclusion 
reached  by  Bateson,  was  one  which  Francis  Galton  had  al- 
ready stated  with  great  clearness  in  1889  {Natural  Inheri- 
tance), viz.,  that  variations  fall  naturally  into  two  classes, 
continuous  and  discontinuous.  Continuous  variations  are 
those  which  are  graded,  the  extremes  being  connected  by  a 
complete  series  of  intermediate  conditions;  discontinuous 
variations  are  such  as  are  separated  by  gaps  in  which  no 
intermediate  stages  occur.  Bateson  believed  that  discon- 
tinuous variations  are  more  important  in  species  formation 
than  are  continuous  ones,  because,  where  variations  are 
discontinuous,  the  action  of  natural  selection  is  greatly  sim- 
plified. In  discontinuous  variation  selection  determines  the 
survival  of  one  or  the  other  of  two  distinct  groups,  since 
intermediates  do  not  occur  and  it  is  unnecessary  to  assign 
selectional  value  to  each  plus  or  minus  gradation  of  an  organ. 
Galton  had  earlier  expressed  the  same  idea,  suggesting  that 
evolution  may  be  like  the  behavior  of  a  polyhedron  when 
pushed.  If  pushed  or  tipped  a  little,  it  returns  to  its  former 
position  of  equilibrium,  merely  oscillating  back  and  forth  on 
the  same  face  as  before.  But  if  it  is  pushed  hard  enough,  it 
rolls  over  on  to  a  new  face  coming  to  rest  in  a  new  position 
of  equilibrium.  Galton  suggested  that  discontinuous  varia- 
tions may  be  species  forming  variations,  stable  from  the  start, 
whereas  slight  or  graded  variations  may  have  no  lasting 
effect,  like  the  oscillations  of  the  polyhedron  on  one  and 
the  same  face.  This  view  was  strongly  supported  a  few 
years  later  by  the  botanist  De  Vries  in  his  theory  of  muta- 
tion (1900-1903). 

Meanwhile  variation  was  being  studied  from  a  new  point 
of  view,  which  we  may  call  biometry.  Francis  Galton  (1889) 
was  the  founder  of  biometry  but  its  full  development  has 


BIOMETRY  57 

been  due  chiefly  to  the  valuable  work  of  Karl  Pearson.  The 
underlying  idea  in  biometry  is  to  apply  to  the  study  of  evolu- 
tion the  precise  quantitative  methods  followed  in  the  study 
of  physics  and  chemistry  with  such  signal  success. 

Biometry  is  the  statistical  study  of  variation  and  heredity. 
It  deals  with  masses,  not  with  individuals,  differing  in  this 
respect  from  the  method  of  Darwin  and  Bateson.  It  seeks 
to  obtain  a  quantitative  estimate,  as  precise  as  possible,  of 
variation  in  one  generation,  and  to  compare  with  this  a 
similar  quantitative  estimate  of  the  next  generation  and  then 
by  comparing  these  to  learn  in  what  direction  evolution  is 
taking  place  and  at  what  rate.  In  some  cases  it  has  at- 
tempted to  discover  the  direction  of  evolution  from  the 
character  of  the  variation  within  a  single  generation. 

Biometry  is  best  adapted  to  deal  with  continuous  varia- 
tion, but  it  has  its  uses  also  in  dealing  with  discontinuous 
variations.  Its  ideal,  to  make  biological  investigation  more 
accurate  and  comprehensive,  is  wholly  commendable.  But 
mere  collection  and  compilation  of  biological  statistics  will 
not  advance  knowledge  unless  brought  into  relation  with 
other  facts  about  living  things,  and  it  is  in  this  respect  chiefly 
that  biometricians  have  sometimes  erred,  drawing  unwar- 
ranted conclusions  from  their  statistical  data. 

Biometry  means  literally  the  measurement  of  living  things. 
It  is  obvious  that  it  can  deal  only  with  characteristics  which 
are  measurable,  such  as  linear  dimensions,  volume,  weight, 
or  number  of  parts.  One  of  the  cases  most  carefully  studied 
by  Galton  was  human  stature.  This  case  illustrates  very 
well  the  methods  and  results  of  biometric  study. 

Measurements  made  at  the  Harvard  gymnasium  of  the 
height  and  weight  of  one  thousand  students  of  ages  eighteen 
to  twenty-five  are  classified  in  Table  1.  In  order  that  the 
number  of  classes  may  not  be  too  great  for  convenient  sta- 
tistical treatment,  height  classes  are  formed  of  3  cm.  each. 
Thus  students  measuring  155,  156,  or  157  cm.  are  all  placed 
in  a  common  class,  whose  middle  value  is  156  cm.  In  dealing 
with  large  numbers,  the  probability  is  that  each  of  the  three 


Number  of 
Individ- 
uals 

180 

If.O 

HO 

120 

100 

80 

CO 

40 

20 

0 

V 

1 
1 

^, 

f 

J 
» 

t 
1 

1 

< 
* 

< 

» 

1 

\ 
t 

1 

\ 

* 

1 

\ 
1 

1 

\ 
( 
\ 

\ 

\ 
\ 
\ 

\ 

< 

f 

1 

1 
i 

f 
1 

1 
I 

r 

( 

\ 
\ 
\ 
\ 

\ 
% 
( 

% 
1 
1 

# 

1 

t 

1 

1 

t 
( 
1 

f 
/ 

/ 
« 
« 

1 

/ 
1 

/ 
1 
# 

1 

% 
1 

» 

/ 
/ 
/ 

« 

t 

/ 

t 
t 

/ 

4 
% 
% 

ir.C      15»     162     165     168      171      174     177      180     183     186      189     192     195     198 

Height  in  OenlirMtera          r\     \ 

Fig.  9.    Frequency-polygon  and  curve  showing  variation  in  height  of  one  thousand 

Harvard  students  of  ages  1&-S5. 


58 


BIOMETRY  59 

measurements  would  occur  as  frequently  as  either  of  the 
others,  so  that  the  middle  value  would  be  a  fair  representa- 
tive of  the  class  and  could  be  used  in  statistical  computations 
as  the  class  value  with  entire  propriety  and  accuracy.  Weight 
classes  are  also  formed  of  three  kilos  extent  in  classifying  the 
weights.  The  numbers  of  individuals  found  in  each  height 
class  are  shown  in  the  totals  at  the  bottom  of  Table  1.  The 
largest  number  of  individuals  is  found  in  the  class,  173-175 
cm.,  viz.,  188.  On  either  side  of  this  class  the  numbers  of 
individuals  (called  frequencies)  fall  off  steadily  reaching  a 
frequency  of  four  in  the  shortest  class  and  of  one  in  the  tallest 
class.  In  Fig.  9  the  relative  frequencies  of  the  height  classes 
are  shown  graphically,  each  column  of  the  figure  being  pro- 
portional in  altitude  to  the  frequency  of  the  class  which  it 
represents.  This  method  of  representing  variation  is  called 
the  "  method  of  loaded  ordinates."  By  joining  the  tops  of 
the  several  columns  of  the  figure,  as  in  the  dotted  line,  a  so- 
called  variation  curve  is  obtained. 

The  class  with  greatest  frequency  in  a  group  of  variates  is 
called  the  mode,  i.  e.,  the  fashionable  class.  It  has,  of  course, 
the  tallest  ordinate  in  the  variation  figure  (class  174,  Fig.  9). 

A  classification  of  the  same  one  thousand  students  as  re- 
gards weight  is  given  in  the  totals  at  the  right  of  Table  1, 
and  a  graphic  presentation  of  the  same  data  in  Fig.  10.  The 
modal  class  is  that  which  has  as  its  middle  value  sixtv-three 
kilos.  This  has  a  frequency  of  one  hundred  and  fifty-four 
with  the  two  adjacent  classes  almost  as  large  and  more  remote 
classes  diminishing  in  frequency  to  minima  in  classes  forty- 
five  and  one  hundred  and  five.  The  falling  off  is  more  rapid 
to  the  left  than  to  the  right  of  the  modal  class,  so  that  in  all 
there  are  only  six  classes  below  the  mode  but  there  are  four- 
teen classes  in  the  range  of  variation  above  the  mode.  This 
results  in  a  "  skew  "  or  asymmetrical  curve  obtained  by 
joining  the  tops  of  the  ordinates  (dotted  line.  Fig.  10).  The 
variation  curve  for  the  height  measurements  (Fig.  9)  was 
also  slightly  skew,  but  its  skewness  was  much  less  than  that 
of  the  curve  for  weight. 


60 


GENETICS  AND  EUGENICS 


A  variation  curve  which  is  free  from  skewness  resembles 
what  mathematicians  call  a  "  frequency  of  error  "  curve  or 
simply  a  '*  curve  of  error  "  or  ''  normal  curve  "  (Fig.  11). 


Number 

of 
Individ 
uals 
160 

140 

120 

100 

80 

/ 
/ 

/ 

\ 

\ 

-V- 

% 
t 

1 

t 

}■ 

( 
f 
• 
• 

1 

1 

1 

% 

t 
> 

< 
% 

% 

« 
« 

1 

i 

t 

1 
\ 

t 

60 

t 
1 

1 
• 

1 

1 
f 

« 

40 

r 

' 
\ 

\ 

1 
> 

* 
• 
< 
1 

\ 
1 

\ 
\ 

i 
\ 

20 

/ 

1 

0 

> 

K.  . 

45    48     61    54    57    €0    63    66     69    72    75    78    81     84    87    90    93    96    99   102  1051 

Weight  in  Kilograms 

Fig.  10.    Frequency-polygon  and  curve  showing  variation  in  weight  of  one  thousand 

Harvard  students  of  ages  18-25. 


BIOMETRY 


61 


TABLE   1 

Showing  the  Variation  in  Height  and  Weight  and  the  Correlation 

BETWEEN  Height  and  Weight  among  1000  Harvard  Students 

OF  Ages  18-25  Measured  at  the  Harvard  Gymnasium 

IN  the  Years  1914-1916 


Weight 

H 

eight 

in  Centimeters 

ID 

Kilos. 

155- 
157 

15S- 
160 

161- 
163 

164- 
166 

167- 
169 

-  170-  17^  176-  179-  182- 
172     175     178     181     184 

185-  188-  191- 
187     190     193 

194- 
196 

197- 
199 

To- 
tals 

44-  46 

1 

•    * 

1 

47-  49 

1 

,    . 

3 

1 

,    , 

,    , 

,    , 

,  , 

.   . 

•   • 

•    • 

5 

50-  52 

1 

2 

1 

6 

4 

6 

2 

.  . 

,    , 

,    , 

.    , 

22 

53-  55 

1 

4 

8 

15 

12 

8 

7 

2 

,    . 

,    , 

1 

58 

56-  58 

1 

4 

10 

15 

19 

20 

11 

3 

2 

,    , 

85 

59-  61 

1 

5 

8 

22 

43 

25 

21 

11 

4 

2 

142 

62-  64 

1 

2 

8 

9 

31 

39 

29 

21 

10 

2 

2 

154 

65-  67 

1 

2 

10 

21 

25 

39 

30 

18 

4 

,   , 

1 

151 

68-  70 

1 

1 

9 

6 

30 

27 

32 

16 

13 

2 

1 

138 

71-  73 

.    , 

2 

4 

5 

18 

20 

12 

18 

15 

4 

2 

100 

74-  76 

•   • 

1 

4 

11 

15 

6 

7 

9 

6 

1 

60 

77-  79 

•   • 

1 

2 

2 

8 

5 

7 

4 

4 

1 

34 

80-  82 

,    , 

•   , 

•   . 

4 

6 

3 

4 

6 

2 

25 

83-  85 

,   , 

•   , 

,   . 

2 

1 

2 

3 

2 

2 

12 

86-  88 

,    , 

2 

1 

2 

,   , 

2 

,   , 

7 

89-  91 

,   , 

,   , 

.   , 

,   , 

,   , 

1 

1 

2 

92-  94 

,    , 

•  , 

,  . 

1 

1 

•  , 

2 

95-  97 

•   • 

,  , 

,  , 

•  • 

•   • 

•  • 

,    , 

98-100 

a     , 

•  • 

•  • 

«  • 

•  • 

•  • 

•   • 

101-103 

•     • 

,   , 

•  • 

•  • 

•  • 

•  • 

,    , 

104-106 

•  • 

•  • 

•  • 

1 

1 

2 

Totals 

4 

8 

26 

53 

89 

146 

188 

181 

125 

92 

60 

22 

4 

1 

1 

1000 

Mean  height  =  175.33  cm.  (5  ft.  9  in.) 
Mean  weight  =  65.66  kilos.  (144.75  lbs.) 
a  height  =  6.58  cm. 

ff  weight  =  7.84  kilos. 


C  V  height  =  3.76  % 
C  P  weight  =  11.94% 
r  height-weight  =  .54 


It  expresses  the  result  of  the  simultaneous  action  of  several 
independent  causes,  or  contingencies.  If,  for  example,  I  toss 
ten  coins  in  the  air  simultaneously,  it  is  certain  that  each  one 
will  show  uppermost  on  landing  either  a  head  or  a  tail,  but 
the  landing  of  one  coin  does  not  affect  that  of  the  others. 
The  landing  of  each  coin  is  a  separate  contingency.    If  the 


62 


GENETICS  AND  EUGENICS 


coins  are  thrown  several  times  and  a  count  made  of  the 
number  of  heads  following  each  throw  and  these  results  are 
then  combined  and  plotted  we  shall  get  a  frequency  of  error 
curve  about  the  number  five  which  will  be  the  most  frequent , 


Fig.  11.    "  Frequency  of  error  "  or  "  normal  "  curve      M,  mode.    Q,  Q',  quartile;   one-half 
the  area  of  the  figure  lies  between  Q  and  Q'.    After  Lock. 

{.  e.,  the  modal  result,  heads  being  of  the  same  frequency  as 
tails.    See  Fig.  12  and  Table  2. 

Biometry  has  established  the  fact  that  biological  variation, 
when  measurable,  is  commonly  of  the  frequency  of  error 


TABLE  2 
Probable  Results  of  Tossing  Ten  Coins  Simultaneously.  (After  Lock) 


Heads 
10 
9 
8 
7 
6 
5 


and 


Tails 

0  . 

1  . 

2  . 

3  . 

4  . 

5  . 


Relative  Probability 

1 

10 

45 

120 

210 

252 


Heads 
4 
3 
2 
1 
0 


Tails     Relative  Probability 


and 


6. 
7. 
8. 
9. 
10. 


210 

120 

45 

10 

1 


type,  which  means  that  it  must  be  the  result  of  several  inde- 
pendent contingencies  or  causes.  Some  of  these  causes  are 
doubtless  environmental,  others  are  due  to  heredity.  Their 
combined  action  is  to  produce  variation  of  the  frequency  of 
error  type. 


BIOMETRY 


63 


The  action  of  several  heredity  factors  which  are  indepen- 
dent of  each  other  produces  a  curve  of  the  same  sort;  and 
so  do  several  environmental  factors  independent  of  each 
other;  in  most  cases  of  variation  agencies  of  both  sorts  are 
at  work.  But  in  some  cases  the  causes  which  tend  to  pro- 
duce plus  variation  may  be  stronger  or  weaker  than  those 
which  tend  to  produce  minus  variation.     The  result  is  an 


Tails      I  2  3  45  6  769  10 

Fig.  12.    A  graphic  presentation  of  the  data  contained  in  Table  2.    After  Lock. 

unsymmetrical  or  "  skew  "  variation  curve.  Thus  among 
Harvard  students  the  causes  which  tend  to  produce  variation 
in  weight  above  the  normal  are  apparently  stronger  than 
those  causes  which  tend  to  produce  weight  below  the  nonnal, 
as  is  indicated  by  Fig.  10.  The  same  was  found  to  be  true  still 
more  emphatically  of  adult  males  in  England,  according  to 
data  tabulated  by  Yule. 

In  some  cases,  biological  variation  is  exclusively  in  one 
direction  from  the  mode,  i.  e.,  all  the  causes  of  variation 
which  are  operative  tend  in  one  direction.  Thus  the  common 
buttercup  varies  in  number  of  petals  from  five  upward  but 
very  rarely  in  the  reverse  direction.  Five  is  the  commonest 
or  modal  number,  but  the  observed  variation  curve  is  one- 
sided.   See  Fig.  14,  H  1887. 

It  is  evident  that  to  describe  the  character  of  variation  in 
any  case  it  will  not  suffice  to  name  the  mode;  we  must  also 


64  GENETICS  AND  EUGENICS 

state  whether  the  variation  is  symmetrical  about  the  mode, 
how  extensive  is  its  range,  and  whether  the  majority  of  the 
variates  cluster  closely  about  the  mode  or  are  widely  scat- 
tered. To  express  these  various  features  of  the  variation, 
special  statistical  coefficients  have  been  devised.  It  will 
suffice  for  our  purposes  to  discuss  only  the  more  important 
of  these. 

1 .  The  mean,  or  average,  is  in  a  case  of  symmetrical  varia- 
tion, identical  with  the  mode.  Thus  the  average  height  of 
the  thousand  Harvard  students  (Table  1)  is  close  to  174  mm., 
the  mode.  But  their  average  weight  lies  outside  and  above 
the  modal  weight  class,  because  their  variation  in  weight 
is  decidedly  skew,  more  men  exceeding  66  kilos  in  weight 
than  fall  below  that  weight.  To  find  the  average,  multiply 
the  value  of  each  class  hy  the  number  of  individuals  contain- 
ed in  it,  add  the  products,  and  divide  hy  the  entire  number  of 
individuals. 

2.  Average  Deviation  and  Standard  Deviation.  Two  sets  of 
variates  having  the  same  mode  and  mean  may  nevertheless 
differ  widely  in  their  variability,  one  being  more  scattered 
than  the  other. 

To  express  the  greater  spread  of  one  curve  as  compared 
with  another,  the  average  deviation,  may  be  employed.  That 
is,  we  may  estimate  how  far,  on  the  average,  an  individual 
taken  at  random  differs  from  the  mean.  This  is  computed  as 
follows :  Find  the  deviation  of  each  class  from  the  mean,  multi- 
ply this  by  the  frequency  of  that  class,  add  the  products,  and 
divide  by  the  entire  number  of  variates.    The  quotient  is  the 

111  D  f 

average  deviation.  Formula  A  D  = in  which  2)  signi- 
fies that  the  sum  is  to  be  taken  of  the  products  indicated, 
D  means  the  deviation  of  each  class  value  from  the  mean  of 
all  variates,  /  means  the  frequency  (number  of  individuals) 
of  each  class,  and  n  means  the  total  number  of  variates 
(individuals).  This  measure  of  variability  is  improved, 
mathematicians  tell  us,  by  the  method  of  least  squares,  i.  e., 
by  squaring  the  deviation  of  each  class,  and  extracting  the 


BIOMETRY  65 

square  root  of  the  final  quotient.    To  distinguish  it  from  the 
average  deviation,  this  is  called  the  standard  deviation.    Its 

•    It  forms  a  measure  of  the  degree  of 

scatter  of  the  variates.    This  measure  is  expressed  in  the  same 
units  as  were  employed  in  measuring  the  variates. 

3.  To  compare  one  case  of  variation  with  another  as  re- 
gards degree  of  scatter  of  the  variates,  another  expression  has 
been  devised  which  is  called  the  Coefficient  of  Variation.  It 
is  obtained  by  dividing  the  standard  deviation  by  the  mean. 

Formula,  CV  =  — 77 It  is  an  abstract  number  expressing 

the  variability  in  per  cent  of  the  mean. 

Judged  by  their  coefficients  of  variability.  Harvard  stu- 
dents are  found  to  be  more  variable  in  weight  than  in  height, 
the  respective  coefficients  (C  V)  for  height  and  weight  being 
3.76  and  11.94.    See  Table  1. 

4.  Another  important  tool  of  the  biometrician  should  be 
mentioned,  viz.,  the  coefficient  of  correlation,  which  is  a 
measure  of  the  extent  to  which  one  character  varies  in 
agreement  with  another. 

In  order  to  obtain  a  coefficient  of  correlation  a  set  of 
observations  may  be  classified  simultaneously  as  regards 
two  characteristics.  Thus  we  might  inquire  is  there  any 
correlation  between  the  height  and  the  weight  of  men,  and  if 
so  how  much  ?  Are  tall  men  on  the  whole  heavier  than  short 
ones  or  vice  versa  ?  To  determine  this  matter  we  must  first 
obtain  observations  on  the  height  and  weight  of  the  same 
individuals.  The  observations  may  then  be  classified  in  a 
correlation  table  (as  in  Table  1),  which  is  made  by  ruling 
paper  into  squares  and  entering  the  observations  on  height 
in  vertical  columns,  and  the  observations  on  weight  in  hori- 
zontal rows,  or  vice  versa.  An  individual  156  cm.  in  height 
and  weighing  48  kilos  will  be  entered  in  the  square  at  which 
column  156  and  row  48  intersect;  an  individual  of  the  same 
height  but  ten  kilos  heavier  will  be  recorded  in  the  third 
square  below,  and  so  on.    When  all  the  observations  have 


66  GENETICS  AND  EUGENICS 

been  entered  in  the  table,  we  may  proceed  to  calculate  ^  a 
coefficient  of  correlation  which  will  be  a  measure  of  the 
extent  to  which  men  vary  in  weight  as  they  vary  in  height. 
Its  numerical  value  will  lie  between  0  and  1. 

It  is  evident  that  the  correlation  would  be  most  complete  if 
men  invariably  increased  in  weight  as  they  increase  in  height. 
The  entries  in  the  table  would  then  be  distributed  in  a  single 
diagonal  row  running  across  the  table  from  its  upper  left-hand 
corner  to  its  lower  right-hand  corner.  We  should  infer  that 
in  such  a  case  the  two  completely  correlated  phenomena  were 
due  to  the  same  causes  or  contingencies  exactly.  Our  numer- 
ical coefficient  of  correlation  would  in  such  a  case  be  -f-  1. 

In  reality  such  correlation  as  this  rarely,  if  ever,  occurs  in 
biological  material.  We  know  that  men  of  the  same  height 
vary  in  weight  and  vice  versa.  For  weight  does  not  depend 
upon  height  alone  but  also  upon  width  and  thickness  and 
specific  gravity.  It  does  however  depend  somewhat  upon 
height,  and  so  our  table  would  show  incomplete  correlation, 
which  would  be  expressed  by  a  coefficient  less  than  1  but 
greater  than  0. 

^  The  coefficient  of  correlation  is  calculated  by  the  formula 

r  = . 

in  which  r  is  the  coefficient  of  correlation,  Di  and  Dy  are  the  deviations  of  each 
observed  group  of  individuals  from  the  respective  means  of  height  and  weight,  S  sig- 
nifies that  the  sum  of  the  products  indicated  is  to  be  taken,  n  is  the  total  num- 
ber of  individuals  observed,  and  <Tx  and  Cy  are  the  standard  deviations  for  height 
and  weight  respectively.  To  express  in  the  form  of  a  rule  the  procedure  to  be 
followed  in  calculating  the  coefficient  of  correlation  between  (say)  height  and 
weight:  First  find  the  average  height  and  the  average  weight  of  all  individuals  ob- 
served, then  their  standard  deviation  in  height  and  their  standard  deviation  in 
weight.  Next  determine  for  each  square  of  the  table  its  deviation  from  the  aver- 
age height  and  average  weight  respectively.  Find  the  product  of  these  two  devia- 
tions (regarding  signs)  and  multiply  it  by  the  number  of  individuals  recorded 
in  the  square  under  consideration.  After  such  a  product  as  this  has  been  found 
for  every  square  in  the  table,  the  products  are  to  be  added  (regarding  signs)  and 
this  sum  is  to  be  divided  by  the  product  of  the  two  standard  deviations  times  the 
total  number  of  individuals  observed.  There  are  several  short-cuts  by  which  the 
calculation  as  here  described  may  be  shortened  or  simplified.  For  a  description  of 
these  the  reader  is  referred  to  the  special  works  of  C.  B.  Davenport  (1904),  Eugene 
Davenport  (1907),  and  Yule  (1912). 


BIOMETRY  67 

In  the  table  the  entries  would  show  a  tendency  to  group 
themselves  about  the  diagonal,  but  there  would  be  a  con- 
siderable scattering  of  entries  in  squares  not  lying  in  the 
diagonal.    Compare  Tables  1  and  3. 

If  men  in  general  did  7iot  increase  in  weight  as  they  increase 
in  height,  but  actually  grew  lighter  as  they  grow  taller,  then 
we  should  find  a  negative  value  for  the  coefficient  of  correla- 
tion. Cases  of  this  kind  are  occasionally  met  with,  but  they 
are  of  no  importance  since  by  rearrangement  of  the  correla- 
tion table  (as  by  reversing  the  order  of  the  grades  for  one 
character)  a  negative  result  may  always  be  converted  into  a 
positive  one  of  like  magnitude.  The  essential  thing,  which  a 
coefficient  of  correlation  does,  is  to  show  whether  two  ob- 
served phenomena  are  or  are  not  causally  related  to  each 
other.  Any  result  other  than  0  indicates  that  the  two  sets 
of  phenomena  are  so  related,  and  the  size  of  the  coefficient 
indicates  the  extent  to  which  they  are  causally  related,  up  to 
a  value  of  +  1  which  would  indicate  that  they  are  due  to 
identical  causes. 

In  biometry  the  correlation  table  has  found  two  principal 
uses  (1)  to  show  what  parts  or  processes  of  an  organism 
vary  in  unison  and  to  what  extent  they  so  vary  and  (2)  to 
measure  heredity.  Examples  of  the  first  use  are  the  relation 
between  height  and  weight  in  man  already  discussed  and 
the  relation  between  one  skeletal  dimension  and  another, 
as  skull  length  and  femur  length,  which  in  rabbits  have  a 
correlation  of  0.76,  or  the  lengths  of  femur  and  humerus, 
which  in  rabbits  show  a  correlation  of  0.86.  See  Table  3. 
The  correlation  values  for  corresponding  bone  measurements 
in  men  are  very  similar.  If  the  correlation  between  two 
parts  is  known,  it  is  possible  from  a  knowledge  of  the 
magnitude  of  one  of  them  to  predict  the  magnitude  of  the 
other,  with  an  accuracv  indicated  bv  the  coefficient  of  corre- 
lation.  If  for  instance  the  correlation  between  femur  and 
humerus  is  0.86  and  I  know  the  femur  length  of  an  individ- 
ual, I  can  estimate  his  humerus  length  with  an  accuracy  of 
about  86  per  cent. 


68 


GENETICS  AND  EUGENICS 


The  second  use  of  the  correlation  coefficient  is  still  more 
important,  viz.,  to  measure  the  strength  of  heredity.  It 
affords  a  means  of  comparing  the  strength  of  a  character  in 
successive  generations  and  of  thus  measuring  its  heredity. 
Thus  the  amount  of  white  on  the  body  of  piebald  rats  is  a 
variable  character  (Fig.  125)  to  some  extent  inherited.  The 
resemblance  between  parents  and  offspring  in  grade  of  white- 
ness as  shown  in  Table  4  is  about  23  per  cent,  the  correlation 
coefficient  in  this  case  being  0.233.  Pearson  found,  for  his 
human  data,  the  height  of  father  and  son  to  have  a  correla- 
tion of  0.514;  between  brother  and  brother  he  found  the 
correlation  to  be  0.511,  figures  which  indicate  the  strong 
inheritance  of  size  differences  in  man. 


TABLE  3 

Correlation  Table  showing  the  Rel.\tion  between  Femur-length 
AND  Humerus-length  IN  370  Rabbits,     r  =  0.857 


From  MacDov 

rell.  Appendix,  Table  16, 

Hnmpriis 

Femur,  Length  in  mm. 

Length  in  mm.      'j 

'6-       78- 
77         79 

SC- 
SI 

S2- 
83 

84-       86-       88- 
85         87         89 

90- 
91 

92- 
93 

94- 
95 

Totals 

60-61 

1  2 

2  16 

9 

1 

13 

51 

13 

1 

4 

62-63 

4 
32 
52 
10 

1 

35 

64-65 

4 

96 

66-67 
68-69 
70-71 
72-73 

47         4       .. 

29       29        4 

3       13       13 

1         4 

•    • 

4 
1 

3 

2 

116 
73 
33 
10 

74-75 

2 

76-77 

78-79 



1 

1 

Totals 

3        27 

79 

99 

83      47       21 

5 

5 

1 

370 

Probable  error  is  a  measure  of  the  reliability  of  a  statistical 
conclusion.  The  need  of  such  a  measure  rests  on  the  fact 
that  the  number  of  observations  on  which  the  conclusion  rests 
is  finite,  that  is  the  number  of  observations  is  smaller  than 
the  class  concerning  which  generalization  is  made.    For  ex- 


BIOMETRY 


69 


ample,  if  I  knew  the  height  of  each  member  of  a  college 
class  I  could  calculate  the  absolute  average  height  of  the 
class  without  any  possible  inaccuracy,  if  the  arithmetical 
operations  were  free  from  mistakes.  But  if  I  want  to  know 
the  average  height  of  students  in  the  entire  college  and  have 
only  the  measurements  of  a  particular  class  on  which  to  base 
an  opinion,  it  is  obvious  that  my  conclusion  is  possibly 
erroneous.  Perhaps  I  have  not  a  fair  sample  of  the  students 
of  the  college  as  regards  height.  Obviously  the  larger  my 
class  the  less  probable  is  any  error  in  my  conclusion.  If  my 
class  included  half  or  more  than  half  of  all  the  men  in  the 
University  (unselected  as  to  size)  the  probability  of  an  error 
through  random  sampling  would  be  small;  and  if  it  included 
all  men  in  the  University,  the  probability  of  error  would 
disappeai;. 

TABLE  4 

Correlation  Table  used  as  a  Measure  op  Heredity.     The  Character 

Studied  is  the  Relative  Amount  of  White  in  the  "  Hooded  " 

Pattern  of  Piebald  Rats,     r  =  0.233. 

From  Castle  and  Phillips,  Table  11. 


Grade  of 

Grade  of  Offspring 

Totals 

Parents 

2f 

3 

3J 

31 

31 

4 

4J 

4i 

4! 

5 

31 

2 

7 

2 

•    • 

11 

31 

2 

7 

17 

87 

162 

41 

11 

3 

3 

.  , 

333 

4 

,   , 

3 

2 

25 

87 

65 

24 

6 

1 

1 

214 

4i 

,    , 

3 

3 

16 

49 

27 

8 

2 

2 

.  . 

110 

4i 

,   , 

,   , 

. . 

2 

13 

5 

3 

1 

1 

.  . 

25 

41 

1 

3 

4 

Totals 

2 

13 

22 

132 

319 

143 

46 

12 

7 

1 

697 

What  statisticians  call  the  probable  error  is  a  pair  of  values 
one  larger  than  the  calculated  value,  one  smaller,  the 
chances  being  even  that  the  true  value  lies  inside  or  outside 
the  limits  of  these  values. 

To  understand  the  significance  of  this  statement,  consider 
for  a  moment  the  normal  curve  or  curve  of  error  (Fig.  11). 
On  either  side  of  its  mean  and  mode  (M)  we  may  draw  a  line 


70  GENETICS  AND  EUGENICS 

(Q^  Q')  so  placed  that  between  the  two  Hnes  half  the  area  of 
the  figure  will  be  included.  It  is  obvious  that  an  individual 
taken  at  random  may  fall  in  any  part  of  the  figure,  but  the 
chances  are  even  that  it  will  fall  inside  or  outside  of  the 
probable  error  (Q,  Q')  since  half  the  group  occurs  in  each 
position.    The  probable  error  of  a  determination  of  the  mean 

equals  ±  0.6745  —7=-     Notice  in  accordance  with  this  that 

the  more  individuals  one  observes  the  more  accurate  his  con- 
clusion, i.  e.,  the  less  the  probable  error,  but  not  in  direct 
proportion  to  the  number  observed  but  to  its  square  root. 

The  probable  error  of  the  standard  deviation  is  expressed 
by  the  equation, 

Ea  =^  0.6745  — ^ 

The  probable  error  of  the  coefficient  of  variability  is 
expressed  by 

CV 


Ec=^  0.6745 


V^ 


n 


The  probable  error  of  the  coefficient  of  correlation  is 

expressed  by 

r,        ±  0.6745  (1  -  r^) 
£jr  —  1= 

yjn 

The  probable  error  of  a  determination  of  the  cross-over 
percentage  between  two  linked  characters  is 


P  (1  —  P)  . 

rt  .6745\/ in  which  P  is  the  observed  cross-over 

T  n 

percentage.      (Haldane,  1919.) 


Private  Property  of 

Z.  p.  r^^ETCALF 


iliJi'i'MJiMiWiM)' 


CHAPTER  VI 


THE  MUTATION  THEORY 


The  theory  that  new  races  and  species  originate  discontinu- 
ously  and  not  gradually,  has  received  its  strongest  support 
from  the  work  of  the  Dutch  botanist,  Hugo  de  Vries,  who 
was  one  of  the  pioneers  in  the  recent  revival  of  the  study  of 
evolution  by  experimental  methods. 

De  Vries  began  studying  the  variation  of  species  of  plants 
in  the  field,  transferring  these  variations  to  his  garden  and 
there  subjecting  them  to  selection.  He  found  that  garden 
conditions,  i.  e.,  cultivation  and  improved  nutrition,  in- 
creased variability  as  regards  minor  differences  in  size, 
luxuriance  and  productiveness.  Such  variations,  which  Bate- 
son  calls  continuous,  De  Vries  speaks  of  as  fluctuating.  They 
depend,  he  thinks,  wholly  upon  nutrition  but  do  not  per- 
manently affect  the  specific  type.  This  is  stable,  like  Galton's 
polyhedron  resting  securely  on  one  of  its  faces.  Its  fluctua- 
tions due  to  nutrition  are  like  the  oscillations  of  the  poly- 
hedron. No  permanent  change  results  from  them.  De 
Vries  indeed  appears  to  think  that  selection  acting  upon 
fluctuations  {i.  e.,  upon  continuous  variations)  may  change 
the  average  condition  of  the  race,  but  that  such  changes  will 
not  persist  unless  maintained  by  rigorous  selection.  As  soon 
as  selection  ceases,  he  thinks,  the  race  begins  a  gradual  return 
to  its  former  condition. 

De  Vries  supported  this  view  both  with  data  from  the  his- 
tory of  cultivated  plants  and  with  direct  experiments  of  his 
own.  He  showed  for  example  that  in  the  history  of  the 
cultivation  of  the  sugar  beet,  the  unimproved  race  contained 
(about  sixty  years  ago)  from  7  to  14  per  cent  of  sugar.  Vil- 
morin  after  two  generations  of  selection  of  the  sweetest  beets 
for  seed  obtained  beets  with  21  per  cent  of  sugar.  Since  then 
the  choice  of  individual  seed  beets  according  to  sugar-content 

71 


72 


GENETICS  AND  EUGENICS 


has  become  general.  Often  hundreds  of  thousands  of  beets 
are  tested  at  a  single  factory.  De  Vries  has  plotted  a  varia- 
tion curve  for  forty  thousand  beets  tested  in  1896  at  a  factory 
in  Holland.  The  result  (Fig.  13)  was  a  beautiful  frequency 
of  error  curve  with  its  mode  at  15.5  per  cent.  The  upper 
limit  of  variation  was  21  per  cent,  or  the  same  per  cent  as 


Fig.  13.     Graph  showing  the  variation  in  sugar-content  of  40,000  sugar  beets  tested  at  a 
factory  in  Holland.    (After  De  Vries.)     The  data  are  as  follows: 

Percent  sugar 
12   12.5      13       13.5  14       14.5  15       15.5  16       16.5  17    17.5       18      18.5  19 

Number 

340     635     1,192     2.205     3,597     5,561     7,178     7,829     6,925     4,458     2,233     692     133     14     5 

The  broken  line  shows  the  theoretical  curve  for  (a+  b)*". 

Vilmorin  obtained  after  two  generations  of  selection.  The 
general  average,  to  be  sure,  is  considerably  higher  than  when 
the  selection  began,  but  De  Vries  believes  that  this  is  due  in 
part  to  improved  methods  of  cultivation  and  more  accurate 
methods  of  determining  the  sugar-content.  He  believes  that 
whatever  real  improvement  has  taken  place  is  due  largely 
to  the  elimination  of  the  poorest  sorts  through  selection,  and 
that  these  would  speedily  become  reestablished  if  the  selec- 
tion were  discontinued. 

The  fact  has  only  recently  come  to  light  that  sugar  beets 
are  regularly  cross-pollinated  by  a  minute  insect,  a  species  of 


VIEWS  OF  DE  VRIES  ON  SELECTION 


73 


thrips,  the  plant  being  scarcely  capable  of  self-pollination. 
This  explains  why  constant  selection  is  required  to  maintain 
a  high  standard.  Hybridization  constantly  occurs  and  for 
this  reason  fully  stable  types  cannot  be  obtained. 

De  Vries  is  also  led  to  adverse  conclusions  concerning  se- 
lection as  an  agency  in  producing  racial  changes  by  experi- 
ments of  his  own,  one  of  the  most  extensive  of  which  was  an 


0  7  8  9  10  II  7^         13 

Pig.  14.  Variation  of  the  buttercup  (Ranunculus  bulbosus)  in  number 
of  petals  preceding  and  following  selection.  H  1387,  variation  curve  of 
unselected  race.  E  1891  and  1892,  curves  for  successive  generations  of  the 
selected  race.  A  1891,  curve  for  parent  plants  of  the  1892  generation. 
(After  De  Vries.) 

attempt  to  increase  by  selection  the  number  of  petals  in  the 
common  meadow  buttercup  (Ranunculus  bulbosus).  This 
regularly  has  five-petaled  flowers,  but  an  occasional  flower 
contains  one  or  more  extra  petals.  See  Fig.  14.  When  this 
plant  was  cultivated  in  his  garden,  De  Vries  found  the  aver- 
age number  of  petals  to  be  5,Q.  After  five  successive  selec- 
tions the  average  was  raised  to  8.6,  the  upper  limit  of 
variation  from  eight  to  thirty-one,  and  the  mode  (or  com- 
monest condition)  from  five  to  nine.  De  Vries  concludes  that 
the  change  thus  produced  could  be  maintained  only  by  con- 
tinued selection,  and  that  further  progress  could  probably 
not  be  made.  This  conclusion  seems  to  me  unwarranted,  but 
I  state  it  as  illustrative  of  the  general  view  of  De  Vries,  who 
maintains  that  when  a  permanent  racial  change  occurs  it  is 
due  to  something  different  from  fluctuating  variability,  viz., 
to  a  discontinuous  variation  or  sport,  a  process  which  De  Vries 


74  GENETICS  AND  EUGENICS 

calls  mutation.  Mutation,  he  believes,  involves  a  change  in 
the  nature  of  the  germ-cells,  whereas  fluctuation  involves 
only  effects  due  to  environment.  These  latter  may  indeed 
modify  the  soma,  and  also  the  germ-plasm  temporarily,  but 
not  permanently.  Weismann,  as  we  have  seen,  admits  for 
certain  cases  a  direct  modification  of  the  germ-cells  by  the 
environment,  and  believes  that  such  modifications  when  once 
produced  are  permanent.  De  Vries  on  the  other  hand  is 
much  more  ready  to  admit  modification  of  the  germ-plasm 
by  the  environment,  but  maintains  that  these  modifications 
are  not  permanent.  Permanent  changes  in  the  germ-plasm, 
according  to  De  Vries,  have  no  relation  to  the  action  of  the 
environment.  They  arise  spontaneously  out  of  internal 
conditions  and  are  not  necessarily  adaptive  in  nature.  Most 
of  them  perish  because  not  adaptive  (i.  e.,  beneficial)  in 
character;  only  those  mutations  survive  in  a  ^tate  of  nature 
which  chance  to  be  adaptive.  The  environment  does  not 
cause  mutations,  according  to  De  Vries,  but  only  determines 
what  ones  may  survive.  Evolution  is  thus  due  primarily  to 
internal  causes;  but  its  course  is  guided  by  the  environment, 
which  selects  those  mutations  which  are  capable  of  survival. 

The  Evidences  of  Mutation 

Two  lines  of  evidences  in  favor  of  mutation  may  be  cited, 
one  general,  the  other  special. 

1.  The  occurrence  of  elementary  species.  Among  many  wild 
species  of  plants  there  occur  varieties  quite  distinct  and 
breeding  true,  but  differing  from  each  other  by  such  minor 
characteristics  as  ordinarily  escape  notice.  Thus  in  the 
common  dandelion  a  considerable  number  of  varieties  may 
be  distinguished.  Some  have  narrow  leaves,  some  broad 
leaves;  on  some  the  leaves  are  deeply  notched,  on  others 
almost  entire.  If  we  save  the  seeds  of  any  of  these  peculiar 
individuals  and  plant  them  we  find  that  the  characteristics  of 
the  parent  plant  are  inherited.  They  breed  true  like  distinct 
species,  indeed  they  may  be  regarded  as  little  species  within 
the  dandelion  species.    De  Vries  calls  them  "  elementary  " 


■  «' 

ll 

'  ^B^l 

i 

■  i&-X~.3» /jsV-'Sti  '■,  r-. 

1 

J 

I 

1 

»■ 

■ 

i 

P 

Hi 

Fig.  15.     Lamarckiana. 


Fig.  17.     Lamarckiana. 


Fig.  19.     Oblonga. 


Fig.  16.     Gigas. 


Fig.  18.     Gigas. 


Fig.  '20.     Lata. 


Oenothera  Lamarckiana  and  Some  of  its  Mutants 

Fig.  15,  late  in  season;  16,  at  mid-season;  17-20,  in  rosette  stage  (wintering-over    stage). 

From  cultures  and  photographs  by  Professor  B.  M.  Davis. 


ELEMENTARY  SPECIES  75 

species.  The  same  thing  may  be  observed  in  the  case  of 
violets;  many  distinct  varieties  or  elementary  species  may 
be  recognized  within  the  commonly  recognized  species,  and 
experiment  has  shown  that  these  breed  true. 

Among  cultivated  plants  a  similar  diversity  of  forms  oc- 
curs, especially  among  such  as  are  self -fertilized,  as  for 
example  wheat,  beans  and  peas.  Varieties  differ  in  shape  of 
leaf,  hairiness,  color  of  seed,  fruit  or  flowers,  and  many  other 
characteristics.  Varieties  of  the  same  species  may  in  many 
cases  be  grown  together  in  the  same  field  without  mixing, 
and  even  if  artificially  crossed  may  not  produce  an  inter- 
mediate character  but  one  which  is  distinctive  of  one  parent 
or  the  other.  The  same  thing  is  true  of  our  domesticated 
animals.  Varieties  are  often  discontinuous,  intermediates 
being  unknown.  De  Vries  joins  with  Bateson  in  urging  a  dis- 
continuous origin  for  such  variations  and  brings  forward 
much  experimental  evidence  in  support  of  this  idea.  He 
supposes  that  discontinuous  variations  arise  through  internal 
causes,  that  is  by  mutation. 

2.  "  Mutation  "  in  Oenothera.  For  proof  of  discontinuity 
in  variation  De  Vries  relies  principally  upon  a  specific  case 
which  he  has  studied  for  many  years,  that  of  Lamarck's 
evening  primrose  {Oenothera  Lamarckiana) .  See  Figs.  15-26. 
This  plant  is  supposed  to  be  of  American  origin.  It  is  culti- 
vated in  Europe  (and  to  some  extent  in  America)  in  parks 
and  gardens,  for  its  showy  yellow  flowers.  Here  and  there  it 
has  escaped  from  cultivation  and  grows  wild.  In  this  condi- 
tion De  Vries  found  it  in  an  abandoned  potato  field  near 
Amsterdam.  But  the  plant  has  not  been  found  growing  wild 
in  the  western  hemisphere,  original  home  of  the  Oenotheras. 
For  this  reason  some  naturalists  are  inclined  to  regard  it  as 
of  hybrid  and  old-world  origin. 

The  plant  is  a  biennial,  five  or  six  feet  high  when  fully 
grown,  with  a  stout  branching  stem  bearing  at  the  ends  of  its 
branches  spikes  of  bright  yellow  flowers.  They  open  towards 
evening,  as  the  name,  evening  primrose,  indicates  and  are 
polHnated  by  bees  and  moths.    On  bright  days  their  duration 


76  GENETICS  AND  EUGENICS 

IS  confined  to  one  evening  and  the  following  morning,  but  in 
cloudy  weather  they  may  remain  open  longer. 

^ATien  De  Vries  discovered  this  plant  growing  wild  in  1886 
he  was  struck  by  its  variability.  It  seemed  to  be  producing, 
in  the  isolated  spot  where  he  found  it,  new  species,  the  thing 
for  which  De  Vries  had  long  been  looking.    He  says: 

I  visited  [the  spot]  many  times,  often  weekly  or  even  daily  during  the 
first  few  years,  and  always  at  least  once  a  year  up  to  the  present  time 
[eighteen  years  later].  This  stately  plant  showed  the  long-sought  peculi- 
arity of  producing  a  number  of  new  species  every  year.  Some  of  them  were 
observed  directly  in  the  field,  either  as  stems  or  as  rosettes  [young  plants  in 
their  first  year's  growth].  The  latter  could  be  transplanted  into  my  garden 
for  further  observation,  and  the  stems  yielded  seeds  to  be  sown  under  like 
control.  Others  were  too  weak  to  live  a  sufficiently  long  time  in  the  field. 
They  were  discovered  by  sowing  seed  from  plants  of  the  wild  locality. 

By  these  means  over  a  dozen  new  types  were  discovered 
never  previously  observed  or  described.  De  Vries  has  given 
to  these  distinctive  names;  some  of  them  he  regards  as  true 
species,  others  merely  as  varieties;  the  basis  of  his  distinction, 
an  arbitrary  one,  does  not  concern  us.    The  peculiarity  of  the 

TABLE  5 

Some  Mutants  of  Oenothera  Lamarckiana 

1.  Smooth-leaved  {laevifolia) 

2.  Short-styled  {hrevistylis) 

3.  Dwarf  (nanella) 

4.  Giant  (gigas)  \  . 

_    Ti   J      .      ,  /     7   .        •  \     r  -rrogressive  or  (jam  variations. 

5.  Ked-vemed  {rubrinervis)    J 

•6.   Pale-leaved  (albida)  |  _,    , , 

_    >-.,,        ,  1/71        N      >  reeble  mutants. 

7.   Ublong-leaved  {pblonga)     J 

case  is,  not  that  a  group  of  undescribed  species  or  varieties 
was  found  growing  together,  but  that  they  were  produced 
year  after  year  from  the  seed  of  the  parent  species,  and  from 
their  first  origin  bred  true  (in  most  cases)  to  their  distinctive 
characters. 

One  of  the  mutants  was  distinguished  by  its  smooth  slen- 
der leaves  {laevifolia)  \  another  by  the  short  style  of  its 
flowers  {hrevistylis) ;  a  third  by  its  dwarf  habit  {nanella,  Fig. 
26),  one-fourth  the  height  of  the  parent  species.  All  three 
bred  true  to  these  peculiarities  which  De  Vries  considers  due 


'  Retrogressive  or  Loss  variations. 


Fig,  21.     Lamarckiana. 


Fig.  23.     Oblonga. 


Fig.  25.    Lata. 


Fig. 

22. 

Gigas. 

> 

1 

1 

'.■■J^^^ 

3 

FnB. 

j 

^ 

£ 

P 

Fig.  24.     Scintillans. 


Fig.  26.     Nanella. 


Oenothera  Lamarckiana  and  Some  of  its  Mutants 
Figs.  21-24,  inflorescence  and  leaf  from  base  of  main  stem;    25,  inflorescence  only; 

26,  entire  plant.     (From  Davis.) 


■I 


\< 


EVIDENCES  OF  MUTATION  77 

to  loss  of  something  the  parent  possessed.    For  this  reason 
he  calls  them  *'  retrogressive  varieties." 

Two  very  vigorous  mutants  the  giant  {gig as.  Figs.  16,  18, 
22)  and  the  red-veined  {ruhrinervis)  De  Vries  considers  to  have 
acquired  additional  characters  not  present  in  the  parent,  and 
for  this  reason  he  regards  them  as  genuine  *'  elementary 
species  "  (having  attained  a  new  progressive  characteristic). 
The  giant  is  no  taller  than  its  parent  species  but  much 
stouter,  with  larger  leaves  and  flowers.  Its  cells  contain 
twice  as  many  chromosomes  as  those  of  the  parent  species, 
which  fact  is  considered  very  important  by  some  cytologists. 
A  wide-leaved  mutant  {lata.  Figs.  20,  25)  has  one  extra  chro- 
mosome in  its  nucleus  (14  +  1  =  15). 

The  red-veined  mutant  {ruhrinervis)  has  more  red  on  its 
leaves  and  stems  than  has  the  parent  species;  its  stems  are 
also  more  brittle,  the  bast  fibres  having  thinner  walls. 

Two  other  mutants  are  naturally  feeble,  not  strong  enough 
to  survive  in  a  wild  state.  They  are  albida  (the  pale  whitish 
mutant),  and  ohlonga  (having  oblong  leaves  on  feeble  plants, 
about  half  as  tall  as  the  parent  species).    See  Figs.  19  and  23. 

"  These  seven  new  forms,"  says  De  Vries,  "  which  diverge 
in  different  ways  from  the  parent  type,  were  absolutely  con- 
stant from  seed.  Hundreds  or  thousands  of  seedlings  may 
have  arisen,  but  they  always  come  true  and  never  revert  to 
the  original  0.  Lamar ckiana-type.^*  Several  other  mutants 
have  been  described  by  De  Vries,  among  them  scintillans, 
but  they  are  less  constant  in  character  than  those  already 
mentioned.    Their  behavior  need  not  here  be  considered. 

A  fact  deserving  especial  attention  in  connection  with  De 
Vries'  experiments  is  the  repeated  occurrence  of  the  same 
mutation  year  after  year  in  pedigree  cultures  from  self -fer- 
tilized plants,  showing  that  these  particular  variations  occur 
with  some  regularity. 

Starting  with  nine  plants  transplanted  from  the  field  De 
Vries  carried  a  culture  through  seven  subsequent  generations, 
always  planting  seed  of  Lamarckiana  parents,  with  the  results 
shown  in  Table  6. 


78  GENETICS  AND  EUGENICS 


TABLE  6 

Eight-generation 

Pedigree  Culture  of  Lamarck's 

Evening 

PRTMR 

Genera- 
tion      Gigas 

Albida 

Oblonga 

Rubri-         Lamarcki- 

nervis               ana             Nanella 

Lata 

Scintil- 
lans 

1 

•    • 

•    • 

9 

•    • 

«    • 

2 

•   • 

•    • 

15,000             5 

5 

•    , 

3 

•   • 

•    • 

1           10,000             3 

3 

,    . 

4          1 

15 

176 

8           14,000           60 

73 

1 

5 

25 

135 

20            8,000          49 

142 

6 

6 

11 

29 

3            1,800            9 

5 

1 

7 

•  • 

9 

3,000           11 

•  • 

•  ■ 

8 

5 

1 

1,700           21 

1 

•  • 

The  giant  mutant  was  obtained  only  once,  but  all  the 
others  in  at  least  three  different  generations,  from  Lamarcki- 
ana  parents. 

Without  going  into  the  details  of  the  case,  to  which  De 
Vries  has  devoted  an  entire  volume,  we  may  notice  what  de- 
ductions or  "  laws  "  De  Vries  bases  upon  it. 

1.  New  elementary  species  appear  suddenly  and  attain  full  constancy  at 
once. 

2.  The  same  new  species  are  produced  in  a  large  number  of  individuals. 

This  would,  of  course,  give  them  a  better  chance  and  fuller 
test  in  the  struggle  for  existence  than  if  they  appeared  but 
once. 

3.  Mutahility  is  something  fundamentally  different  from  fluctuating  vari- 
ability. All  organs  and  all  qualities  of  Lamarckiana  fluctuate  and  vary 
in  a  more  or  less  evident  manner,  and  those  which  I  had  the  opportunity  of 
examining  more  closely  were  found  to  comply  with  the  general  laws  of 
fluctuation.  But  such  oscillating  changes  have  nothing  in  common  with 
the  mutations.  Their  essential  character  is  the  heaping  up  of  slight  devia- 
tions around  a  mean,  and  the  occurrence  of  continuous  lines  of  increasing 
deviations,  Hnking  the  extremes  with  this  group.  Nothing  of  the  kind  is 
observed  in  the  case  of  mutations.  There  is  no  mean  for  them  to  be 
grouped  around  and  the  extreme  only  is  to  be  seen,  and  it  is  wholly  un- 
connected with  the  original  type.  It  might  be  supposed  that  on  closer 
inspection  each  mutation  might  be  brought  into  connection  with  some 
feature  of  the  fluctuating  variability.  But  this  is  not  the  case.  The  dwarfs 
are  not  at  all  the  extreme  variants  of  structure,  as  the  fluctuation  of  the 
height  of  the  Lamarckiana  never  decreases  or  even  approaches  that  of  the 
dwarfs.  There  is  always  a  gap.  The  smallest  specimens  of  the  tall  type 
are  commonly  the  weakest,  according  to  the  general  rule  of  the  relationship 
between  nourishment  and  variation,  but  the  dwarfs  according  to  this  same 
rule  are  of  course  the  most  robust  specimens  of  their  group. 


EVIDENCES  OF  MUTATION  79 

Fluctuating  variability,  as  a  rule,  is  subject  to  regression.  The  seeds 
of  the  extremes  do  not  produce  an  offspring  which  fluctuates  around  their 
parents  as  a  center,  but  around  some  point  on  the  line  which  combines 
their  attributes  with  the  corresponding  characteristic  of  their  ancestors, 
as  Vilmorin  has  put  it.  No  regression  accompanies  mutation,  and  this 
fact  is  perhaps  the  completest  contrast  in  which  these  two  great  tj-pes  of 
variability  are  opposed  to  each  other. 

The  offspring  of  my  mutants  are,  of  course,  subject  to  the  general  laws 
of  fluctuating  variabihty.  They  vary,  however,  around  their  own  mean, 
and  this  mean  is  simply  the  tj^De  of  the  new  elementary  species. 

4.    The  mutations  take  'place  in  nearly  all  directions. 

Some  are  larger,  others  smaller  than  the  parent  species; 
some  more  vigorous  and  productive,  others  less  so;  some  are 
more  heavily  pigmented,  others  less  so;  some  can  survive  in 
competition  with  the  parent  form,  others  cannot.  There  is 
no  evidence  of  adaptive  modification,  or  modification  con- 
trolled by  the  environment  for  the  benefit  of  the  species. 
The  variation  is  in  all  directions. 

The  facts  upon  which  De  Vries  bases  these  generalizations 
have  been  verified  in  the  main  by  a  number  of  workers  in 
different  parts  of  the  world,  notably  in  this  country  where 
several  botanists  have  studied  the  seedlings  of  Lamarck's 
evening  primrose.  But  the  facts  are  not  interpreted  in  the 
same  way  by  all  observers. 

One  view  accepts  the  facts  at  their  face  value,  including 
the  regularity  of  the  occurrence  of  the  same  mutation  in 
successive  generations,  and  its  entire  distinctness  from  the 
parent  form,  but  maintains  that  0.  Lamarckiana  is  a  hybrid 
plant,  not  a  pure  species,  and  that  the  so-called  mutation  is 
only  a  new  illustration  of  the  splitting  up  of  a  hybrid  into  new 
forms,  many  of  which  are  constant,  a  thing  which  is  known 
frequently  to  occur  following  hybridization. 

In  support  of  this  view  it  may  be  said  that  0.  Lamarckiana 
has  not  been  found  growing  wild  in  this  country,  its  supposed 
place  of  origin,  though  careful  search  has  been  made  for  it. 
On  the  other  hand  0.  Lamarckiana  has  for  many  years  been 
growing  wild  in  certain  English  stations,  notably  on  the  sand 
hills  north  of  Liverpool,  and  there  are  good  reasons  for  be- 
lieving that  the  Lamarckiana  first  brought  out  by  seedsmen 


80  GENETICS  AND  EUGENICS 

about  the  year  1860  may  have  come  from  some  English  lo- 
cality. The  fact  that  several  species  of  Oenothera  are  known 
to  have  been  in  England  previous  to  this  date  suggests  that 
Lamarckiana  may  have  arisen  through  the  crossing  of  other 

forms. 

In  this  connection  it  is  of  interest  to  note  that  a  hybrid 
has  been  synthesized  by  Davis  from  a  cross  of  0 .  franciscana 
with  0,  biennis,  which  is  essentially  indistinguishable  in  its  sys- 
tematic characters  from  0.  Lamarckiana.  Furthermore  this 
hybrid  behaves  like  Lamarckiana  in  producing  two  classes  of 
progeny  when  crossed  with  certain  wild  species  as  described 
in  the  next  paragraph.  This  Lamarckiana-\\ke  hybrid, 
which  has  been  given  the  name  of  neo-Lamarckiana,  in  the 
fourth  generation  bred  true  for  about  one-third  of  its  pro- 
geny and  therefore  gave  a  very  much  larger  percentage  of 
variants  than  Lamarckiana,  but  its  seed  fertility  was  very 
much  higher,  which  may  account  for  the  fact.  At  this  stage 
in  the  investigation  neo-Lamarckiana  presents  a  breeding 
behavior  at  least  similar  to  that  of  Lamarckiana  and  it  will 
be  a  matter  of  interest  to  see  whether  in  later  generations  the 
resemblance  may  not  become  more  marked. 

Another  adverse  view  of  De  Vries'  theory,  with  less  concern 
as  to  the  origin  of  0.  Lamarckiana,  maintains  that  however  it 
originated  it  is  clearly  not  pure  genetically;  if  not  actually  a 
hybrid  of  recent  origin,  it  at  least  has  the  genetic  character  of 
a  hybrid  and  hence  the  regularity  of  its  mutations.  For  hy- 
bridization, as  we  shall  see,  is  a  sure  means  of  producing  new 
and  stable  varieties.  Hybridization  experiments  made  by 
De  Vries  and  repeatedly  confirmed  by  others  show  that  in 
every  generation  0.  Lamarckiana  produces  different  kinds  of 
fertile  gametes.  In  particular,  it  forms  two  classes  of  hy- 
brids, "  twin  hybrids,"  in  approximately  equal  numbers,  in 
crosses  with  certain  wild  species,  as  do  several  of  the  wild 
species  in  crosses  with  each  other,  so  that  it  is  evident  that 
0.  Lamarckiana,  as  well  as  some  wild  species  of  Oenothera, 
have  the  variability  characteristic  of  hybrids.  Even  those 
which  seem  to  breed  true,  and  which  do  breed  true  when 


EVIDENCES  OF  MUTATION  81 

self -pollinated,  may  give  a  variable  progeny  in  crosses,  and 
they  seem  to  breed  true  merely  because  certain  classes  of  their 
progeny  are  too  feeble  to  survive.  For  in  some  cases  only  a 
fractional  part  of  the  seeds  produced  contain  embryos 
capable  of  survival. 

According  to  the  views  expressed  above,  Oenothera  Lamar ck- 
iana  is  best  interpreted  as  an  impure  or  hybrid  species  which 
only  breeds  true  in  a  relatively  high  degree  because  of 
extensive  sterility,  which  eliminates  large  numbers  of  gametes 
and  zygotes  that  differ  from  the  germinal  cells  which  repro- 
duce the  Lamarckiana  type.  The  "  mutants  "  come  from 
occasional  seeds  of  different  types  that  survive  the  heavy 
mortality  which  renders  sixty  per  cent  or  more  of  the  seeds 
infertile  and  about  fifty  per  cent  of  the  pollen  grains  abortive. 
If  this  is  the  correct  explanation  of  the  peculiar  breeding 
behavior  of  Lamarckiana,  this  plant  is  very  far  from  being 
representative  of  a  pure  species,  as  De  Vries  assumed  it  to  be, 
and  is  hardly  suitable  material  for  experiments  designed  to 
give  evidence  of  mutation. 

Even  if  we  reject  this  explanation  and  consider  that  the 
mutability  of  the  evening  primrose  has  no  causal  relation  to 
its  hybridity,  it  by  no  means  follows  that  mutation  is  a 
general  method  of  origin  of  new  varieties  and  species  among 
animals  and  plants,  which  is  the  thesis  of  De  Vries.  In  recent 
years  the  expression  "mutation  theory"  has  been  used  in  a 
sense  very  different  from  that  in  which  De  Vries  originally 
used  it,  and  implying  merely  the  origin  of  new  and  stable 
organic  forms  by  change  in  single  inheritance  factors  (genes) , 
whether  these  produce  striking  variations  (sports)  or  varia- 
tions so  minute  as  to  be  scarcely  observable.  This  form  of 
mutation  theory  will  be  discussed  in  a  later  chapter.  To  the 
mutation  theory  of  De  Vries,  as  a  general  theory  of  evolution, 
it  seems  to  be  a  fatal  objection  that  such  mutation  as  it  re- 
cognizes is  not  general  in  occurrence.  Crosses  of  species  or 
varieties  as  found  in  the  wild  state  more  often  reveal  the 
existence  of  numerous  minute  genetic  differences  than  a 
single  or  a  few  striking  differences. 


CHAPTER  \TI 

THE  PIONEER  PLANT  HYBRIDIZERS:    THE  DISCOVERY 
AND  REDISCOVERY  OF  MENDEL'S  LAW 

While  De  Vries  was  engaged  in  his  studies  of  the  evening 
primrose  he  hit  upon  an  idea  far  more  important,  as  most 
biologists  now  beheve,  than  the  idea  of  mutation,  though 
De  Vries  himself  both  then  and  since  has  seemed  to  regard 
it  as  of  only  minor  importance.  He  called  this  the  "  law  of 
the  splitting  of  hybrids.''  The  same  law,  it  is  claimed,  was 
independently  discovered  about  the  same  time  by  two  other 
botanists,  Correns  in  Germany,  and  Tschermak  in  Austria. 
Further,  historical  investigations  made  by  De  Vries  showed 
that  the  same  law  had  been  discovered  and  clearly  stated 
many  years  previously  by  an  obscure  naturalist  of  Briinn, 
Austria,  named  Gregor  Mendel,  and  we  have  now  come  to 
call  this  law  by  his  name,  Mendel's  law.  Mendel  was  so  little 
known  when  his  discovery  was  published  that  it  attracted 
little  attention  from  scientists  and  was  soon  forgotten,  only 
to  be  unearthed  and  duly  honored  years  after  the  death  of  its 
author.  Had  Mendel  lived  forty  years  later  than  he  did,  he 
would  doubtless  have  been  a  devotee  of  biometry,  for  he  had 
a  mathematical  type  of  mind  and  his  discovery  of  a  law  of 
hybridization  was  due  to  the  fact  that  he  applied  to  his 
biological  studies  methods  of  numerical  exactness  which  he 
had  learned  from  algebra  and  physics.  In  biology  he  was  an 
amateur,  being  a  teacher  of  the  physical  and  natural  sciences 
in  a  monastic  school  at  Briinn.  Later  he  became  head  of  his 
monastery  and  gave  up  scientific  work,  partly  because  of 
other  duties,  partly  because  of  failing  eyesight. 

The  subject  of  plant  hybridization  had  received  consider- 
able attention  from  botanists  for  a  century  before  it  was 
taken  up  by  Mendel  and  the  law  of  the  splitting  of  hybrids 
which  was  discovered  by  Mendel  and  rediscovered  by  De 

8^ 


THE  FIRST  PLANT  HYBRIDIZER  83 

Vries  had  narrowly  escaped  discovery  at  the  hands  of  their 
predecessors.  There  was  lacking  only  the  numerical  exact- 
ness of  a  Mendel  or  the  clear-sighted  analysis  of  a  De  Vries 
to  bring  to  light  the  rule  governing  the  splitting  of  hybrids. 

By  a  hybrid  we  understand  an  organism  produced  by  the 
crossing  of  two  distinct  species  or  varieties  of  plant  or  animal, 
i,  e,,  an  organism  which  has  an  individual  of  one  species  or 
variety  as  its  mother  and  an  individual  of  a  different  species 
or  variety  as  its  father.  At  times  and  by  certain  naturalists 
a  distinction  has  been  made  between  the  offspring  of  a  species 
cross  and  that  of  a  variety  cross,  the  term  hybrid  being 
limited  to  the  progeny  of  a  species  cross,  and  the  term  mon- 
grel being  used  to  designate  the  progeny  of  a  variety  cross. 
But  it  has  been  found  quite  impossible  to  distinguish  species 
from  varieties  sharply,  for  Darwin  showed  that  varieties  may 
be  only  incipient  species,  and  that  no  definition  can  be 
framed  of  variety  which  will  not  also  include  species  and 
vice  versa.  Accordingly  at  present  we  use  the  terms  species 
and  variety  in  a  relative  sense  only.  The  differences  which 
exist  between  species  are  supposed  to  be  either  more  numer- 
ous or  greater  in  degree  than  those  which  exist  between 
varieties.  The  terms  to  the  majority  of  biologists  imply 
nothing  more  than  this.  If  we  cannot  distinguish  species 
from  varieties,  it  is  obvious  that  we  cannot  distinguish  the 
products  of  a  species-cross  from  the  products  of  a  variety- 
cross,  and  so  at  present  all  cross-bred  offspring,  whether  of 
species  or  varieties,  are  called  hybrids.  The  same  law  of 
splitting  applies  to  all,  as  we  shall  see. 

The  pioneer  plant  hybridizer  was  Joseph  (Gottlieb)  Kol- 
reuter  (1733-1806)  who  between  the  years  1760  and  1766 
carried  out  the  first  series  of  systematic  experiments  in  plant 
hybridization  which  had  ever  been  undertaken.  The  more 
important  features  of  Kolreuter's  work  have  been  thus 
summarized  by  Lock,  pp.  150-155. 

These  experiments  not  only  established  with  certainty  for  the  first  time 
the  fact  that  the  seeds  of  plants  are  produced  by  a  sexual  process  com- 
parable with  that  known  to  occur  in  animals,  but  also  led  to  a  knowledge 


84  GENETICS  AND  EUGENICS 

of  the  general  behaviour  of  hybrid  plants,  which  was  scarcely  bettered  until 
Mendel  made  his  observations  a  century  afterwards. 

Kolreuter  found  that  the  hybrid  offspring  of  two  different  plants  gener- 
ally took  as  closely  after  the  plant  which  yielded  the  pollen  as  after  that 
upon  which  the  actual  hybrid  seed  was  borne.  Indeed,  he  found  that  it 
made  little  or  no  difference  in  the  appearance  of  the  hybrid  which  of  the 
parental  species  was  the  pollen-parent  (male),  and  which  the  seed-parent 
(female)  —  that  is  to  say,  in  the  case  of  plants  the  result  of  reciprocal 
crosses  is  usually  identical.  Thus,  for  the  first  time  it  was  definitely  shown 
that  the  pollen-grain  plays  just  as  important  a  part  in  determining  the 
characters  of  the  offspring  as  does  the  ovule  which  the  pollen-grain  fer- 
tilizes. This  was  a  wholly  novel  idea  in  Kolreuter's  time,  and  the  fact  was 
scarcely  credited  by  his  contemporaries. 

Kolreuter  had  no  means  of  discovering  that  the  contents  of  a  single 
pollen-grain  unite  with  the  contents  of  a  single  ovule  in  fertilization.  But 
he  ascertained  by  experiments  that  more  than  thirty  seeds  might  be  made 
to  ripen  by  the  application  of  between  fifty  and  sixty  pollen-grains  to  the 
stigma  of  a  particular  flower,  so  that,  if  he  had  had  any  hint  of  the  actual 
microscopic  processes  of  fertilization,  he  would  have  been  quite  prepared 
for  the  more  fundamental  discovery. 

Kolreuter,  indeed,  believed  that  the  act  of  fertilization  consisted  in  the 
intimate  mingling  together  of  two  fluids,  the  one  contained  in  the  pollen- 
grain,  and  the  other  secreted  by  the  stigma  of  the  plant.  The  mingled 
fluids,  he  supposed,  next  passed  down  the  style  into  the  ovary  of  the  plant, 
and  arriving  at  the  unripe  ovules,  initiated  in  them  those  processes  which 
led  to  the  formation  of  seeds.  In  this  belief  Kolreuter  simply  followed  the 
animal  physiologists  of  his  time,  who  looked  upon  the  process  of  fertiliza- 
tion in  animals  as  taking  place  by  a  similar  mingling  of  two  fluids.  Now 
that  we  know  that  fertilization  consists  essentially  in  the  intimate  union 
of  the  nuclei  of  two  cells,  one  of  which,  in  the  case  of  plants,  is  the  ovum 
contained  within  the  ovule,  whilst  the  other  is  represented  by  one  of  a 
few  cells  into  which  the  contents  of  the  pollen-grain  divide,  we  can  under- 
stand more  clearly  the  bearing  of  Kolreuter's  observation.  And  it  is 
greatly  to  this  eminent  naturalist's  credit  that  he  succeeded  in  carrying 
out  his  observations  with  so  much  accuracy,  when  the  full  meaning  of 
those  observations  was  of  necessity  hidden  from  his  comprehension. 

Kolreuter  was  the  first  to  observe  accurately  the  different  ways  in  which 
pollen  can  be  naturally  conveyed  to  the  stigma  of  a  flower.  This  may 
take  place  either  by  the  p>ollen-grains  falling  directly  upon  the  stigma,  or 
by  the  agency  of  the  wind,  or,  lastly,  the  pollen  may  be  carried  by  insects 
visiting  the  flowers.  And  he  recognized  many  features  characteristic  of 
flowers  apt  to  be  fertilized  in  one  or  other  of  these  ways  in  particular. 
Thus  he  was  aware,  for  example,  of  the  nature  and  use  of  the  nectar  which 
so  many  flowers  produce  —  namely,  that  it  is  the  substance  from  which 
the  bees  —  by  far  the  most  diligent  visitors  of  flowers  —  obtain  their 
honey. 

Curiously  enough,  Kolreuter  was  not  aware  of  the  existence  of  any 
natural  wild  hybrid  plants.    But  he  was  quite  right  in  contending  that 


Fig.  26a.  The  first  artificially  produced  plant  hybrid  and  its  parents.  A,  Nicotiana  paniculata;  B, 
N.  rustica  var.  humilis;  C,  Fi  hybrid  between  A  and  B;  D,  individual  flowers  of  the  hybrid  (middle), 
of  N.  paniculata  (right) ,  and  of  N.  rustiea  (left).  Photographs  by  Prof.  E.  M.  East,  from  his  repetition 
of  Kolreuter's  pioneer  experiment. 


LOCK  ON  THE  WORK  OF  KOLREUTER  85 

supposed  examples  of  such  hybrids  required  for  their  substantiation  the 
experimental  proof,  which  could  only  be  afforded  by  making  actual  artifi- 
cial crosses  between  the  putative  parent  species. 

The  first  hybrid  made  artificially  by  Kolreuter  was  obtained  in  ITtiO 
by  applying  the  pollen  of  Nicotiana  paniculata  to  the  stigma  of  Nicotiana 
rustica.  The  hybrid  offspring  of  this  cross  showed  a  character  intermediate 
between  those  of  the  two  parent  species  in  almost  every  measurable  or 
recognizable  feature,  with  a  single  notable  exception.  This  exception  was 
afforded  by  the  condition  of  the  stamens  and  of  the  pollen  grains  pro- 
duced by  the  hybrids.  These  organs  were  so  badly  developed  that  in  all 
the  earlier  experiments,  self-fertilization  of  the  hybrid  plants  yielded  no 
good  seed  at  all,  nor  were  the  pollen  grains  of  the  hybrid  any  more  effec- 
tive when  applied  to  the  stigmas  of  either  of  the  parent  species.  On  the 
other  hand,  when  pollen  from  either  parent  was  applied  to  the  stigmas  of 
the  hybrid  plants,  a  certain  number  of  seeds  capable  of  germination  was 
obtained,  although  this  number  was  much  smaller  than  in  the  case  of 
normal  fertilization  of  either  parent  species.  This  partial  sterility,  affect- 
ing in  particular  the  stamens  and  the  pollen  which  they  produce,  is  a 
feature  common  to  the  majority  of  hybrids  between  different  natural 
species.  Many  such  hybrids,  indeed,  are  altogether  sterile,  so  that  a 
further  generation  cannot  in  any  way  be  obtained  from  them.  On  the 
other  hand,  the  members  of  different  strains  or  varieties  which  have  arisen 
under  cultivation  \Tield,  as  a  rule,  when  crossed  together  offspring  which 
are  perfectly  fertile. 

In  subsequent  years  Kolreuter  was  able  to  obtain  a  very  few  self-ferti- 
lized offspring  from  hybrids  of  the  same  origin  as  the  above.  The  resulting 
plants  were  described  as  resembling  their  hybrid  parent  so  closely  as  to  be 
practically  indistinguishable  from  it. 

The  offspring  obtained  by  crossing  the  hybrid  plants  with  pollen  from 
either  parent  showed  in  each  case  a  form  more  or  less  intermediate  between 
that  of  the  original  hybrid  and  that  of  the  parent  species  from  which  the 
pollen  was  derived.  But  the  plants  were  not  all  alike  in  this  respect,  some 
of  them  being  much  more  like  the  parent  species  than  others,  and  some, 
again,  varying  in  other  directions.  There  were  also  considerable  differ- 
ences between  the  different  individuals  in  respect  of  fertiHty.  so  that  some 
of  the  plants  were  more  and  some  less  sterile  than  the  original  hybrids. 
Also,  there  was  some  tendency  to  the  production  of  malformations  of  the 
flowers  and  other  parts. 

One  of  the  most  noted  of  Kolreuter's  experiments  was  that  which  con- 
sisted in  repeatedly  crossing  a  hybrid  plant  with  one  of  the  parent  species 
from  which  the  hybrid  was  derived.  By  continuing  to  pollinate  the  mem- 
bers of  one  generation  after  another  with  the  pollen  of  the  same  parent 
species,  plants  were  at  last  arrived  at  which  were  indistinguishable  from 
the  parent  in  question.  We  shall  return  to  this  fact  later  on,  when  the 
reader  will  be  in  a  position  to  appreciate  its  importance  more  fully. 

Kolreuter  found  that  the  result  of  reciprocal  crosses  is  usually  identical 
—  that  is  to  say,  the  offspring  obtained  by  fertilizing  a  plant  A  with  ]H)llon 
from  a  plant  B  are  not  to  be  distinguished  from  those  obtained  when  B  is 


86  GENETICS  AND  EUGENICS 

fertilized  with  the  pollen  of  A.  But  the  two  opposite  processes  of  fertili- 
zation are  not  always  equally  easy  to  carry  out.  An  extreme  instance  of 
this  circumstance  was  met  with  in  the  case  of  the  genus  Mirabilis.  Mirab- 
ilU  jalapa  was  easily  fertilized  with  pollen  from  M.  longiflora.  During 
eight  years  Kolreuter  made  more  than  two  hundred  attempts  to  effect  the 
reverse  cross,  but  without  success. 

It  was  shown  by  Kolreuter  that  hybrids  between  different  races  or 
varieties  of  the  same  species  are  usually  much  more  fertile  than  hybrids 
obtained  by  crossing  distinct  species.  Indeed,  he  believed  that  varieties 
of  a  single  species  were  in  all  cases  perfectly  fertile  together,  whilst  hybrids 
between  species  always  showed  some  degree  of  sterility.  But  in  this  case 
Kolreuter  based  his  definition  of  a  species  upon  the  very  point  at  issue, 
and  when  he  found  forms,  which  other  botanists  regarded  as  good  species, 
to  be  perfectly  fertile  together,  he  immediately  regarded  them  as  being 
only  varieties  of  a  single  species. 

One  curious  point  is  worth  noting  in  this  connection.  Five  varieties 
of  Nicotiana  tahacum  were  found  to  be  perfectly  fertile  with  one  another, 
but  when  crossed  with  Nicotiana  glutinosa  one  of  them  was  found  to  be 
distinctly  less  sterile  than  the  rest. 

Another  interesting  point  observed  by  Kolreuter  was  the  fact  that 
hybrid  plants  often  exceed  their  parents  in  luxuriance  of  growth.  Upon 
this  fact,  as  we  shall  see  later  on.  Knight  and  afterwards  Darwin  based 
theoretical  conclusions  of  considerable  importance  in  connection  with  the 
problem  of  sex. 

To  pick  out  the  salient  features  of  the  foregoing  account 
we  may  notice: 

1.  That  Kolreuter  established  the  occurrence  of  sexual 
reproduction  in  plants  by  showing  that  hybrid  offspring  in- 
herit equally  from  the  pollen  plant  and  the  seed  plant. 

2.  He  showed  that  hybrids  are  commonly  intermediate 
between  their  parents  in  nearly  all  characters  observed,  such 
for  example  as  size  and  shape  of  parts. 

3.  Many  hybrids  are  partially  or  wholly  sterile,  especially 
when  the  parents  are  very  dissimilar  (belong  to  widely  dis- 
tinct species).  Such  hybrids  often  exceed  either  parent 
species  in  size  and  vigor  of  growth. 

4.  Kolreuter  did  not  observe  the  regular  splitting  of  hy- 
brids which  Mendel  and  De  Vries  record,  but  some  of  his 
successors  did,  particularly  Thomas  Knight  (1799)  ^  and  John 
Goss  (1822)  1  in  England  who  were  engaged  in  the  crossing  of 
garden  peas  with  a  view  to  producing  more  vigorous  and 

*  For  a  fuller  account  of  the  work  of  these  early  plant  hybridizers,  see  Lock. 


NAUDIN  MENDEL'S  FORERUNNER  87 

productive  varieties,  and  Naudin  (1862)  in  France  who 
made  a  comprehensive  survey  of  the  facts  of  hybridization 
in  plants  and  came  very  near  to  expressing  the  generalization 
which  Mendel  reached  four  years  later.  He  pointed  out  the 
significance  of  the  fact  first  observed  by  Kolreuter  that  hy- 
brids may  be  brought  back  to  the  form  of  either  parent  by 
repeated  crossing  with  that  parent.  Naudin  supposes  that 
the  potentialities  of  each  species  are  contained  in  its  pollen 
and  ovules  and  the  potentialities  of  both  species  are  present 
together  in  the  hybrid.  If  species  A  is  fertilized  by  species  B, 
the  hybrid  contains  potentialities  AB.  Naudin  supposes 
that  these  potentialities  may  segregate  from  each  other  in 
the  pollen  grains  and  ovules  of  the  hybrid  plant.  An  ovule 
A  of  such  a  hybrid  plant,  if  fertilized  by  pollen  of  the  pure 
species  A,  will  form  a  plant  of  exactly  the  same  nature  as 
pure  species  A.  This  idea  of  the  segregation  of  potentialities 
in  the  germ-cells  of  the  hybrid  was  adopted  by  Mendel.  He 
added  to  it  the  conception  that  the  segregation  applies  to 
single  potentialities  or  characteristics  rather  than  to  all  the 
potentialities  of  a  species  at  once,  and  the  result  is  what  we 
call  Mendel's  law.  Like  all  great  discoveries  it  was  not  made 
out  of  hand,  nor  as  the  result  of  one  man's  work  alone. 
Mendel  added  one  final  touch  to  the  work  of  his  predecessors 
as  summarized  by  Naudin,  and  the  result  was  that  hybridi- 
zation became  for  the  first  time  an  orderly  and  understand- 
able process,  capable  of  throwing  light  on  normal  heredity. 


CHAPTER  VIII 

MENDEL'S  LAW  OF  HEREDITY  ILLUSTRATED  IN 

ANIMAL  BREEDING 

Mendel's  law  may  best  be  explained  with  the  aid  of  ex- 
amples, which  will  be  chosen,  for  convenience,  from  the 
heredity  of  guinea-pigs.  If  a  guinea-pig  of  pure  race  with 
colored  fur  (say  black)  is  mated  with  a  guinea-pig  having 
uncolored  (white)  fur,  a  so-called  albino,  the  offspring  will 
all  have  colored  fur,  none  being  albinos.  See  Figs.  27-30. 
To  use  Mendel's  terminology,  colored  fur  dominates  in  the 
cross,  while  albinism  recedes  from  view.  Colored  fur  is,  there- 
fore, called  the  dominant  character;  albinism,  the  recessive 
character. 

But  if  now  two  of  the  colored  individuals  produced  by  this 
cross  are  mated  with  each  other,  the  recessive  (albino) 
character  reappears  on  the  average  in  one  in  four  of  their 
offspring  (Fig.  30).  The  reappearance  of  the  recessive  char- 
acter, after  skipping  a  generation,  in  the  particular  propor- 
tion, one  fourth,  of  the  second  generation  offspring,  is  a 
regular  feature  of  Mendelian  inheritance.  It  may  be  ex- 
plained as  follows  (see  Fig.  30a) :  the  gametes  which  united  in 
the  original  mating  of  a  pure  colored  individual  with  an 
albino  must  have  transmitted,  one  color  (C),  the  other 
albinism  (c) .  The  contrasted  characters  were  then  associated 
together  in  the  offspring.  But  color  from  its  nature  domi- 
nated, since  albinism  is  due  apparently  to  the  lack  of  some- 
thing necessary  to  the  formation  of  color,  which  the  other 
gamete  would  supply. 

But  when  the  young  produced  by  this  cross  have  become 
adult  and  themselves  form  gametes,  the  characters,  color  and 
albinism,  will  separate  from  each  other  and  pass  into  differ- 
ent gametes,  since,  as  regards  the  transmission  of  alternative 

88 


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IS 

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CO 
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a 
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■c 
a 
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MENDEL'S  LAW 


89 


characters  like  color  and  albinism,  a  gamete  is  able  to  trans- 
mit only  one,  its  nature  being  simplex. 

Accordingly  a  female  hybrid  will  transmit  the  character, 
color  (C),  in  half  its  eggs,  and  the  contrasted  character,  al- 


Fig.  30a.    Diagram  to  explain  the  inheritance  of  color  (C)  and  albinism  (c)  in  the 

cross  shown  in  Figs.  27-30. 

binism  (c),  in  half  its  eggs.    A  male  hybrid  will  also  transmit 
color  (C)  in  half  its  sperm,  and  albinism  (c)  in  the  other  half. 


90  GENETICS  AND  EUGENICS 

If  the  type  of  egg  which  transmits  color  (C)  is  fertihzed  as 
readily  by  one  type  of  sperm  as  by  the  other,  combinations 
will  result  which  are  either  CC  or  Cc  in  character.  And  if  the 
tj^pe  of  egg  which  transmits  albinism  (c)  is  also  fertilized  as 
readily  by  one  kind  of  sperm  as  by  the  other,  combinations 
will  result  which  are  either  Cc  or  cc  in  character.  Putting 
together  the  results  expected  from  the  fertilization  of  both 
types,  we  get  1  CC  :  2  Cc  :  1  cc,  i.  e.,  one  combination  of  color 
with  color,  two  combinations  of  color  with  albinism,  and  one 
combination  of  albinism  with  albinism;  or  three  combina- 
tions which  contain  color  (and  so  w^ill  show  it)  to  one  combi- 
nation which  lacks  color  and  so  will  be  white.  This  agrees 
with  the  observed  average  result. 

The  albino  individual  may  be  expected  to  transmit  only 
the  albino  character  (c),  never  color  (C),  which  it  does  not 
possess.  Experiment  shows  this  to  be  true.  Albino  guinea- 
pigs  mated  with  each  other  produce  only  albino  offspring. 
But  the  colored  individuals  are  of  two  sorts,  CC  and  Cc  in 
character.  The  CC  individual  is  pure,  so  far  as  its  breeding 
capacity  is  concerned.  It  can  form  only  C  gametes.  But 
the  Cc  individuals  may  be  expected  to  breed  exactly  like 
the  first  generation  hybrids,  w^hich  had  the  same  composition. 
They  will  transmit  color  (C)  in  hg^lf  their  gametes,  albinism 
(c)  in  the  other  half.  Experiment  justifies  these  expectations 
also.  The  test  of  individual  animals  may  readily  be  made  by 
mating  them  one  by  one  with  albinos.  The  pure  colored  in- 
dividuals (CC)  will  produce  only  colored  offspring,  since  they 
transmit  color  (C)  in  all  their  gametes.  But  the  other  and 
more  numerous  class  of  colored  individuals  (Cc)  will  produce 
offspring  part  of  which  will  be  colored  (Cc)  and  the  remainder 
albino  (cc).  The  two  kinds  of  dominant  individuals,  those 
which  breed  true  and  those  which  do  not,  w^e  may  call 
homozygous  and  heterozygous,  following  the  convenient  ter- 
minology of  Bateson.  A  homozygous  individual  is  one  in 
which  like  characters  are  joined  together,  as  CC  or  cc;  a 
heterozygous  individual  is  one  in  which  unlike  characters  are 
joined  together,  as  Cc.    It  goes  without  saying  that  reces- 


ij 


wj 


Fig.  31 


Fig.  32 


Fig.  33  Fig.  34 

Figs.  31-34.  Results  of  a  cross  between  two  varieties  of  guinea-pig  differing  in  two  unit-characters, 
color  and  roughness  of  fur.    Fig.  31,  a  colored  and  smooth-coated  guinea-pig. 

Fig.  32.   An  albino  and  rough-coated  guinea-pig.     Fig.  33.    One  of  the  Fi  young,  colored  and  rough. 

Fig.  34.  A  smooth-coated  albino,  one  of  the  four  varieties  occurring  among  the  F2  young.  The  other 
three  varieties  of  F2  young  are  like  the  parents  and  grandparents  respectively  (Figs.  31-33). 


Fip.  :i5 


Fi^'.  .'>(i 


Fig.  37  Fig-  38 

Figs,  35-38.  Results  of  a  cross  between  two  varieties  of  guinea-pig  differing  in  the  two  unit-characters, 
color  and  length  of  fur.  Fig.  35,  a  colored  and  short-haired  guinea-pig.  Fig.  36,  an  albino  and 
long-haired  guinea-pig.  The  Fi  young  were  colored  and  short-haired  like  the  parent  shown 
in  Fig.  35.  Fig.  37,  a  colored  and  long-haired  guinea-pig.  one  of  the  new  F2  varieties. 
Fig.  38,  an  albino  and  short-haired  guinea-pig,  the  other  new  F2  variety.  The  two  other 
F2  varieties  were  like  the  grandparents  (Figs.  35  and  36). 


4 


MENDEL'S  LAW  91 

sives  are  always  homozygous.  For  they  do  not  contain  the 
dominant  character;   otherwise  they  would  show  it. 

It  will  be  observed  that,  in  the  cross  of  colored  with  albino 
guinea-pigs,  color  and  albinism  behave  as  a  pair  of  alternative 
units  which  may  meet  in  fertilization  but  separate  again  at 
the  formation  of  gametes. 

Mendel's  law  as  illustrated  in  this  cross  includes  three 
principles:  (1)  The  existence  of  unit-characters,  (2)  domi- 
nance, in  cases  where  the  parents  differ  in  a  unit-character, 
and  (3)  segregation  of  the  units  contributed  by  the  respective 
parents,  this  segregation  being  found  among  the  gametes 
formed  by  the  offspring. 

The  principles  of  dominance  and  segregation  apply  to  the 
inheritance  of  many  characteristics  in  animals  and  plants. 
Thus  in  guinea-pigs  a  rough  or  rosetted  coat  (Figs.  32  and  33) 
is  dominant  over  the  ordinary  smooth  coat.  If  a  pure  rough 
individual  is  crossed  with  a  smooth  one,  all  the  offspring  are 
rough;  but  in  the  next  generation  smooth  coat  reappears  in 
one-fourth  of  the  offspring,  as  a  rule.  Again,  in  guinea-pigs 
and  rabbits  a  long  or  angora  condition  of  the  fur  (Figs.  36, 
and  37)  is  recessive  in  crosses  with  normal  short  hair.  All 
the  immediate  offspring  of  such  a  cross  are  short  haired,  but 
in  the  next  generation  long  hair  reappears  in  approximately 
one-fourth  of  the  offspring. 

In  cattle,  the  polled  or  hornless  condition  is  dominant  over 
the  normal  horned  condition;  in  man,  two  jointed  fingers 
and  toes  are  dominant  over  normal  three-jointed  ones. 

In  each  of  the  cases  thus  far  considered  a  single  unit- 
character  is  concerned.  Crosses  in  such  cases  involve  no 
necessary  change  in  the  race,  but  only  the  continuance  within 
it  of  two  sharply  alternative  conditions.  But  the  result  is 
quite  different  when  parents  are  crossed  which  differ  sunul- 
taneously  in  two  or  more  independent  unit-characters.  Cross- 
ing then  becomes  an  active  agency  for  the  production  of  new 
varieties. 

In  discussing  the  crosses  now  to  be  described,  it  will  be 
convenient  to  refer  to  the  various  generations  in  more  pre- 


92  GENETICS  AND  EUGENICS 

cise  terms,  as  Bateson  has  done.  The  generation  of  the 
animals  originally  crossed  will  be  called  the  parental  genera- 
tion (P);  the  subsequent  generations  will  be  called  fihal 
generations,  viz.,  the  first  filial  generation  (Fi),  second  fihal 
(F2),  and  so  on. 

When  guinea-pigs  are  crossed  of  pure  races  which  differ 
simultaneously  in  two  unit-characters,  the  Fi  offspring  are 
all  alike,  but  the  F2  offspring  are  of  four  sorts.  Thus,  when 
a, smooth  colored  animal  (Fig.  31)  is  crossed  with  a  rough 
albino  (Fig.  32),  the  Fi  offspring  are  all  rough  and  colored 
(Fig.  33) ,  manifesting  the  two  dominant  unit-characters,  — 
colored  coat  derived  from  one  parent,  rough  coat  derived 
from  the  other.  But  the  F2  offspring  are  of  four  sorts,  viz., 
(1)  smooth  and  colored,  like  one  grandparent,  (2)  rough  and 
albino,  like  the  other  grandparent,  (3)  rough  and  colored, 
like  the  Fi  generation,  and  (4)  smooth  and  albino,  a  new 
variety  (Fig.  34).  It  will  be  seen  that  the  pigmentation  of 
the  coat  has  no  relation  to  its  smoothness.  The  dark  animals 
are  either  rough  or  smooth,  and  so  are  the  white  ones.  Pig- 
mentation of  the  coat  is  evidently  a  unit-character  indepen- 
dent of  hair  direction,  and  as  new  combinations  of  these  two 
units  the  cross  has  produced  two  new  varieties,  —  the  rough 
colored  and  the  smooth  albino. 

Again,  hair-length  is  a  unit-character  independent  of  hair- 
color.  For  if  a  short-haired  colored  animal  (either  self  or 
spotted,  Fig.  35)  be  crossed  with  a  long-haired  albino  (Fig. 
36),  the  Fi  offspring  are  all  short-haired  and  colored,  but 
the  F2  offspring  are  of  four  sorts,  viz.,  (1)  colored  and  short- 
haired,  like  one  grandparent,  (2)  albino  and  long-haired, 
like  the  other,  (3)  colored  and  long-haired,  a  new  combina- 
tion (Fig.  37),  and  (4)  albino  and  short-haired,  a  second  new 
combination  (Fig.  38). 

Now  the  four  sorts  of  individuals  obtained  from  such  a 
cross  as  this  will  not  be  equally  numerous.  As  we  noticed  in 
connection  with  the  simple  cross  of  colored  with  albino 
gumea-pigs,  dominant  individuals  are  to  the  corresponding 
recessives  as  three  to  one.     Therefore,  we  shall  expect  the 


MENDEL'S  LAW  93 

short-haired  individuals  in  F2  to  be  three  times  as  numerous 
as  the  long-haired  ones,  and  colored  ones  to  be  three  times 
as  numerous  as  albinos.  Further,  individuals  which  are  both 
short-haired  and  colored  should  be  3  X  3  or  nine  times  as 
numerous  as  those  which  are  neither  short-haired  nor  colored. 
The  expected  proportions  of  the  four  classes  of  F2  offspring 
are  accordingly  nine  short  colored  :  three  long  colored  :  three 
short  albino :  one  long  albino,  a  proportion  which  is  closely 
approximated  in  actual  experience. 

The  Mendelian  theory  of  independent  unit-characters  ac- 
counts for  this  result  fully.  No  other  hypothesis  has  as  yet 
been  suggested  which  can  account  for  it.  Suppose  that  each 
independent  unit  has  a  different  material  basis  in  the  gamete. 
Let  us  represent  the  material  basis  of  hair-length  by  a  circle, 
that  of  hair-color  by  a  square;  then  combinations  and  re- 
combinations arise  as  shown  in  Fig.  39.  The  composition  of 
the  gametes  furnished  by  the  parents  is  shown  in  the  first  line 
of  the  figure;  that  of  an  Fi  zygote,  in  the  second  line; 
that  of  the  gametes  formed  by  Fi  individuals  in  the  third 
line.  S  meets  s  and  C  meets  c  in  fertilization  to  form  an  Fi 
individual  duplex  and  also  heterozygous  as  regards  hair- 
length  and  hair-color,  but  these  units  segregate  again  as  the 
gametes  of  the  Fi  individuals  are  formed,  and  it  is  a  matter 
of  chance  whether  or  not  they  are  associated  as  originally, 
S  with  C  and  s  with  c,  or  in  a  new  relationship,  s  with  C  and 
S  with  c.  Hence  we  expect  the  Fi  individuals  to  form  four 
kinds  of  gametes  all  equally  numerous:  SC,  sc,  sC,  and  Sc. 
By  chance  unions  of  these  in  pairs  nine  kinds  of  combinations 
become  possible,  and  their  chance  frequencies  will  be  as 
follows : 

Short  Colored  Long  Colored  Short  Albino  Long  Albino 

1  SSCC  1  ssCC  1  SScc  1  sscc 

2  SSCc  2  ssCc  2  Sscc 
2  SsCC 

4  SsCc 


9  3  3  1 

Four  of  these  combinations,  including  nine  individuals,  will 
show  the  two  dominant  characters,  short  and  colored;   two 


94 


GENETICS  AND  EUGENICS 


classes,  including  three  individuals,  will  show  one  dominant 
and  one  recessive  character,  viz.,  colored  and  long;  two 
more  classes,  including  three  individuals,  will  show  the  other 
dominant  and  the  other  recessive  character,  viz.,  short  and 
albino;  and  lastly,  one  class,  including  a  single  individual, 
will  show  the  two  recessive  characters,  long  and  albino.    The 


Fig.  39.    Diagram  to  explain  the  simultaneous  and  independent  inheritance  of  colored  fur  (C)  and 

short  hair  (S)  in  the  cross  shown  in  Figs.  35-38. 

four  apparent  classes,  or,  as  Johannsen  calls  them,  pheno- 
types,  will  accordingly  be  as  9:3:3:1. 

One  individual  in  each  of  these  four  classes  will,  if  mated 
with  an  individual  like  itself,  breed  true,  for  it  is  homozygous, 
containing  only  like  units.  The  double  recessive  class,  long 
albino,  of  course  contains  only  homozygous  individuals,  but 
in  each  class  which  shows  a  dominant  unit,  heterozygous 
individuals  outnumber  homozygous  ones,  as  2  :  1,  or  8  :  1. 

Now  the  breeder  who  by  means  of  crosses  has  produced  a 
new  type  of  animal  wishes,  of  course,  to  "  fix  "  it,  —  that  is. 


Fig.  40 


Fig.  41 


Fig.  42 


Fig.  43 


Fig.  44 


Fig.  45 


Fig.  46 


Fig.  47 


Figs.  40-47.  Results  of  a  crass  between  varieties  of  guinea-pig  differing  in  three  unit-characters,  color, 
length  and  roughness  of  fur.  Fig.  40,  the  colored,  short-haired  and  smooth  parent.  Fig.  41,  the  albino, 
iong-haired  and  rough  parent.  Fig.  42,  one  of  the  Fi  young,  colored,  short-haired  and  rough.  Figs. 
43-47,  five  new  varieties  occurring  among  the  F2  young.  Fig.  43,  colored,  long-haired  and  rough. 
Fig.  44,  colored,  long-haired  and  smooth.  Fig.  45,  albino,  short-haired  and  rough.  Fig.  46,  albino, 
long-haired  and  smooth.  Fig.  47,  albino,  short-haired  and  smooth.  Throe  other  F2  varieties  were 
like  the  parents  and  grandparents  respectively  (Figs.  40-42) . 


MENDEL'S  LAW  95 

to  obtain  it  in  a  condition  which  will  breed  true.  He  must, 
therefore,  obtain  homozygous  individuals.  If  he  is  dealing 
with  a  combination  which  contains  only  recessive  characters, 
this  will  be  easy  enough,  for  such  combinations  are  invari- 
ably homozygous.  His  task  will  become  increasingly  diffi- 
cult, the  more  dominant  characters  there  are  included  in  the 
combination  which  he  desires  to  fix. 

The  most  direct  method  for  him  to  follow  is  to  test  by 
suitable  matings  the  unit-character  constitution  of  each  in- 
dividual which  shows  the  desired  combination  of  characters, 
and  to  reject  all  which  are  not  homozygous.  In  this  way  a 
pure  race  may  be  built  up  from  individuals  proved  to  be  pure. 
Such  a  method,  however,  though  sure,  is  slow  in  cases  where 
the  desired  combination  includes  two  or  more  dominant  unit- 
characters,  for  it  involves  the  application  of  a  breeding  test 
to  many  dominant  individuals,  most  of  which  must  then  be 
rejected.  It  is,  therefore,  often  better  in  practice  to  breed 
from  all  individuals  which  show  the  desired  combination,  and 
eliminate  from  their  offspring  merely  such  individuals  as  do 
not  show  that  combination.  The  race  will  thus  be  only 
gradually  purified,  but  a  large  stock  can  be  built  up  much 
more  quickly. 

We  may  next  discuss  a  cross  in  which  three  unit-character 
differences  exist  between  the  parents,  instead  of  two.  If 
guinea-pigs  are  crossed  which  differ  simultaneously  in  three 
unit-characters,  color,  length,  and  direction  of  the  hair,  a 
still  larger  number  of  phenotypes  is  obtained  in  F2,  namely, 
eight.  A  cross  between  a  short-haired,  colored,  smooth 
guinea-pig  (Fig.  40)  and  one  which  was  long-haired,  albino, 
and  rough  (Fig.  41)  produced  offspring  in  Fi  which  were 
short-haired,  colored,  and  rough  (Fig.  42),  these  being  the 
three  dominant  characters,  two  derived  from  one  parent,  one 
from  the  other.  The  F2  offspring  were  of  eight  distinct  t>T)es, 
two  like  the  respective  grandparents,  one  like  the  Fi  indi- 
viduals (parents),  and  the  other  five  new,  shown  in  Figs.  43- 
47.  The  largest  of  the  eight  apparent  classes  {phenotypes) 
was  the  one  which  manifested  the  three  dominant  charac- 


96 


GENETICS  AND  EUGENICS 


ters,  short,  colored,  and  rough,  which  had  been  the  exclusive 
Fi  type  (Fig.  42) ;  the  smallest  class  was  the  one  which  mani- 
fested the  three  recessive  characters,  long,  albino,  and  smooth 


Fig.  48.    Diagram  to  explain  the  simultaneous  and  independent  inheritance  of  short  (S) 
colored  (C)  and  rough  (R)  fur  in  the  cross  shown  in  Figs.  40-47. 

(Fig.  46).  Theoretically  these  two  classes  should  be  to  each 
other  as  27  :  1.  Of  the  twenty-seven  triple  dominants, 
twenty-six  should  be  heretozygous.  The  triple  recessive 
would  of  course  be  fully  homozygous. 


MENDEL'S  LAW  97 

A  comparison  of  this  case  with  the  one  just  previously 
described  shows  what  an  increasingly  difficult  thing  it  is  to 
fix  types  obtained  by  crossing,  as  the  number  of  dominant 
characters  in  the  selected  type  increases.  On  the  theory  of 
unit-characters  the  gametic  combinations  and  segregations 
in  this  cross  are  as  shown  in  Fig.  48.  The  nature  of  the 
gametes  formed  by  the  parents  crossed  is  shown  in  the  first 
row;  the  composition  of  the  Fi  individuals,  immediately  be- 
low. In  the  two  lower  rows  are  shown  four  different  sorts  of 
gametic  splittings  which  may  occur  in  Fi  individuals,  pro- 
ducing thus  eight  different  kinds  of  gametes. 

If,  as  suggested,  the  Fi  individuals  produced  in  this  cross 
form  eight  different  kinds  of  gametes,  each  of  these  kinds 
should,  when  united  with  a  gamete  having  the  same  consti- 
tution as  itself,  produce  a  homozygous  and  so  true-breeding 
zygote  of  a  different  variety,  making  in  all  eight  true-breeding 
varieties.  Experiment  has  shown  that  in  reality  eight  such 
varieties  are  produced  in  F2.  It  is  therefore  evident  that  the 
crossing  of  varieties  which  differ  from  each  other  by  unit- 
characters  becomes,  under  the  operation  of  Mendel's  law,  a 
ready  means  of  producing  other  new  varieties  different  from 
those  crossed,  and  that  the  number  of  such  new  varieties 
capable  of  production  in  this  way  increases  rapidly  with  every 
additional  unit-character  difference  between  the  parent 
varieties  which  are  crossed. 


CHAPTER  IX 

SOME  MENDELIAN  TERMS  AND  THEIR  USES 

In  describing  Mendelian  heredity  it  is  convenient  for  brevity 
to  use  technical  terms,  some  of  which  are  already  in  general 
use  among  biologists,  but  others  of  which  have  been  framed 
to  meet  needs  not  previously  existing.  The  significance  of 
these  the  reader  must  keep  clearly  in  mind,  for  which  reason 
it  seems  best  brieflv  to  define  them. 

A  gamete  is  a  reproductive  cell  capable  of  uniting  with 
another  reproductive  cell  to  form  a  new  individual.  In  all 
the  higher  animals  and  plants  the  gametes  which  are  capable 
of  union  in  pairs  are  of  two  unlike  sorts,  eggs  and  sperms. 

An  egg-cell  (capable  of  fertilization)  is  the  larger,  non- 
motile  gamete,  produced  by  the  female  parent,  when  the 
parents  are  sexually  different. 

A  sperm  is  the  smaller  gamete,  commonly  motile,  and  pro- 
duced by  the  male  parent,  when  the  parents  are  sexually 
different.  Exceptions  to  the  motility  of  sperms  occur  in  the 
Crustacea  among  animals  and  in  all  but  the  lowest  of  the 
flowering  plants.  In  the  lowest  flowering  plants  motile 
sperms  are  found  in  the  pollen-tube,  but  in  the  ordinary 
flowering  plants  the  two  gametes  which  are  produced  in  the 
pollen-tube  are  non-motile.  The  pollen-tube  itself  transports 
them  by  its  growth  toward  the  egg-cell  of  the  plant. 

A  zygote  results  from  the  union  of  two  gametes  in  fertiliza- 
tion, an  egg  with  a  sperm.  It  is,  potentially  or  actually, 
a  new  individual  produced  by  a  sexual  process  (union  of 
gametes) . 

A  homo-zygote  results  from  the  union  of  gametes  which 
transmit  the  same  Mendelian  character,  as  black  joined  with 
black,  or  white  joined  with  white. 

A  hetero-zygote  results  from  the  union  of  gametes  which 
transmit  alternative  Mendelian  characters,  as  black  united 
with  white. 

98 


MENDELIAN  TERMS  99 

Mendelian  characters  exist  in  contrasted  pairs  which  are 
alternatives  of  each  other,  as  black  and  white,  rough  and 
smooth,  long  and  short.  A  gamete  may  from  its  nature  trans- 
mit only  one  of  a  pair,  either  black  or  w^hite,  but  not  both. 
Its  nature  is  simplex.  A  zygote  is  duplex  in  nature;  it  may 
contain  a  character  twice  represented  (when  it  is  a  homo- 
zygote) ,  or  contain  both  a  character  and  its  alternative  (when 
it  is  a  heterozygote) .  The  same  zygote  may  be  a  homozygote 
as  regards  one  character  (say  hair-color)  and  a  heterozygote 
as  regards  another  (say  hair-length). 

Unit-character  or  unit-factor  or  gene.  Such  characters  of 
animals  and  plants  as  follow  Mendel's  law^  in  heredity,  i.  e., 
are  inherited  as  independent  units,  are  often  called  unit- 
characters.  But  it  has  been  shown  in  numerous  cases  that 
an  independent  factor,  which  follows  Mendel's  law  in  trans- 
mission, may  affect  or  condition  the  inheritance  of  a  supposed 
unit-character,  without  itself  producing  any  other  discover- 
able effect.  Thus  the  agouti  (or  yellow-ticked)  character  of 
the  fur  of  rodents  is  not  developed  unless  along  with  the 
other  genetic  factors  which  produce  a  black  or  a  brown  coat, 
a  particular  "  agouti  "  factor  is  present;  yet  we  have  no 
other  evidence  of  the  existence  of  this  factor,  except  the  form 
which  the  black  or  brown  coat  assumes  when  this  factor  is 
inherited.  But  it  can  be  shown  unmistakably  that  the  in- 
heritance of  this  unseen  factor  is  that  of  an  independent 
Mendelian  character. 

Some  have  sought  to  avoid  the  difficulty  presented  by  such 
cases  by  making  a  distinction  between  unit-characters  and 
unit-factors,  the  former  being  the  recognized  morphological 
or  physiological  parts  or  properties  of  the  organism,  the 
latter  their  hypothetical  determiners.  But  this  distinction 
is  of  doubtful  utility,  since  the  only  objective  evidence  which 
we  possess  that  unit-characters  exist  is  the  occurrence  of 
classes  among  the  F2  individuals  and  their  numerical  fre- 
quencies. But  this  same  evidence  also  forms  our  only  indi- 
cation that  determiners  exist.  In  fact  the  "  unit-characters  " 
about  which  we  talk  are  the  hypothetical  determiners.    For 


100  GENETICS  AND  EUGENICS 

no  one  familiar  with  Mendelian  phenomena  would  venture 
to  classify  the  anatomical  parts  or  physiological  processes  of 
an  organism  as  unit-characters  in  heredity  merely  because 
they  are  distinct  anatomical  parts  or  distinct  physiological 
processes. 

The  head,  the  hand,  the  stomach,  stomach-digestion,  — 
these  are  not  unit-characters  so  far  as  any  one  knows.  But  if 
a  race  without  hands  were  to  arise  and  this  should  Mendelize 
in  crosses  with  normal  races,  then  we  should  speak  of  a  unit- 
character  or  unit-factor  for  "  hands,"  loss  of  which  or  varia- 
tion in  which  had  produced  the  abnormal  race.  But  in  so 
doing  we  should  refer  not  to  the  hand  as  an  anatomical  part  J 
of  the  body  nor  to  the  thousand  and  one  factors  concerned 
in  its  production  but  merely  to  one  hypothetical  factor  to 
which  we  assign  the  failure  of  the  hand  to  develop  in  a 
particular  case.  It  is  immaterial  whether  we  call  this  a  imit- 
character  or  unit-factor  or  use  both  terms  inter-changeably, 
but  it  would  be  a  mistake  to  suppose  that  they  refer  to  differ- 
ent things  or  that  one  is  less  abstract  than  the  other.  Histori- 
cally the  term  unit-character  has  priority,  though  factor  seems 
better  to  express  the  abstract  and  purely  hypothetical  nature 
of  the  conception  involved.  The  application  of  the  term 
unit-character  at  first  to  certain  agencies  which  were  later 
found  to  be  complex  led  to  the  coining  of  a  new  term  (unit- 
factor)  to  apply  to  the  newly  recognized  simpler  agencies.  If 
this  process  were  to  be  continued  indefinitely  we  should  have 
to  invent  a  new  set  of  terms  for  every  step  in  advance  in 
Mendelian  analysis.  It  seems  better  to  discard  earlier  and 
imperfect  analyses  as  knowledge  advances  but  not  to  multi- 
ply technical  terms  needlessly  when  no  new  conception  is 
involved. 

Parental  and  filial  generations.  The  manifestation  of  Men- 
delian characters  is  often  very  different  in  successive  gener- 
ations, for  which  reason  it  is  necessary  to  have  a  convenient 
means  of  designating  the  different  generations  concerned. 
The  significant  generation  from  which  reckoning  should  be 


^ 


MENDELIAN  TERMS  101 

made  is  that  in  which  hybridization  occurs,  i.  e.,  in  which 
parents  of  unHke  character  are  mated  with  each  other.  Tliis, 
following  Bateson,  we  may  call  the  parental  generation  or  P 
generation.  Subsequent  generations  are  called  filial  genera- 
tions (abbreviated  F)  and  their  numerical  order  is  indicated 
by  a  subscript,  as  first  filial  (Fi),  second  filial  (F2),  etc. 
\Mien  pure  races  are  crossed  the  first  filial  generation  (Fi) 
is  usually  as  uniform  in  character  as  the  parental  races.  Any 
striking  lack  of  uniformity  in  Fi  may  be  taken  as  prima  facie 
evidence  that  one  or  other  of  the  parent  races  is  impure 
(heterozygous  for  one  or  more  characters) .  It  is  in  the  F2 
generation  that  recombinations  are  formed  of  the  characters 
in  which  the  parent  races  differ  from  each  other.  The  num- 
bers of  classes  of  individuals  obtained  in  F2  and  their  numeri- 
cal proportions  are  the  significant  features  which  indicate 
how  many  Mendelizing  factors  distinguish  the  parental  races 
and  w^hat  their  nature  is,  whether  dominant  or  recessive. 

The  members  of  contrasted  pairs  of  Mendelian  characters 
are  known  as  allelomorphs,  i.  e.,  alternative  forms.  For  ex- 
ample, colored  and  albino  coat  are  allelomorphs  among 
guinea-pigs,  as  also  are  rough  and  smooth,  long  and  short. 
The  dominant  allelomorph  is  that  one  which  is  expressed  in 
the  heterozygote;  the  recessive  allelomorph  is  that  one  which 
is  not  expressed  in  the  heterozygote.  It  follows  that  domi- 
nant allelomorphs  are  regularly  expressed  in  Fi  while  recessive 
allelomorphs  are  as  regularly  suppressed  in  that  generation, 
but  that  both  of  them  find  expression  in  F2,  though  domi- 
nants exceed  recessives  in  F2  as  three  to  one. 

For  the  simplification  of  inheritance  formulae,  Mendelian 
factors  are  commonly  designated  by  letters  of  the  alphabet, 
members  of  the  same  allelomorphic  pair  being  designated  by 
the  same  letter,  a  capital  being  used  for  the  dominant  allelo- 
morph, a  small  letter  for  the  recessive  allelomorph.  It  will 
assist  the  reader  to  choose  letters  which  suggest  descriptive 
names  of  the  characters  involved.  Thus  for  the  agouti  factor 
we  may  use  A,  for  its  recessive  allelomorph  a;  for  the  color 


102  GENETICS  AND  EUGENICS 

factor  we  may  use  C,  and  for  its  recessive  allelomorph  (found 
in  albinos)  c,  etc. 

Though  a  gamete,  from  its  simplex  nature,  may  never 
contain  more  than  a  single  allelomorph,  and  a  zygote,  from 
its  duplex  origin,  may  never  contain  more  than  two  allelo- 
morphs, the  same  race  may  contain  three  or  more  variations 
which  belong  in  the  same  allelomorphic  series;  i.  e.,  which 
are  allelomorphs  of  each  other.  In  such  a  race,  a  gamete  may 
transmit  any  one  of  the  series,  and.  a  zygote  may  contain 
any  two,  but  never  more.  In  such  cases  the  original  termi- 
nology of  Mendel,  which  involved  the  use  of  capitals  and 
small  letters,  becomes  inadequate,  and  it  has  been  deemed 
advisable  to  use  in  its  stead  a  numerical  or  descriptive  sub- 
script. Thus  four  allelomorphic  conditions  of  the  color  factor 
found  among  guinea-pigs  have  been  designated  C,  Cd,  Cr, 
and  Ca  respectively. 

In  calculating  the  result  to  be  expected  from  a  particular 
cross  it  is  obviously  necessary  to  consider,  not  the  number  of 
characters  which  the  parents  possess,  but  only  the  number  in 
which  they  differ,  since  as  regards  these  only  will  heterozy- 
gotes  be  formed  in  Fi,  to  be  followed  by  the  production  of 
new  homozygous  combinations  in  F2.  Our  inheritance 
formulae  therefore  will  contain  only  differential  factors  but 
the  student  must  not  fall  into  the  error  of  supposing  these  to 
be  the  only  factors  concerned.  A  thousand  factors  held  in 
common  by  the  parents  are  doubtless  involved  to  every  one 
in  which  the  parents  are  observed  to  differ.  But  factors  held 
in  common  are  incapable  of  demonstration  by  the  method  of 
experimental  breeding.  A  factor  reveals  itseK  only  by  its 
disappearance  or  alteration  in  gametes  produced  by  one  of 
the  parents  crossed. 

Both  from  Mendelian  theory  and  from  the  experience  of 
practical  breeders,  it  is  clear  that  individuals  which  look 
alike  often  do  not  breed  alike.  Hence  it  is  useful  to  recognize 
(with  Johannsen)  a  "  phenotype  "  as  including  all  individuals 
which  look  or  seem  alike,  and  in  counter  distinction  to  this 


I 


MENDELIAN  TERMS  103 

to  recognize  a  "  genotype  "  which  includes  only  such  indi- 
viduals as  breed  alike,  i.  e.,  which  produce  the  same  kind  or 
kinds  of  gametes.  A  single  phenotype  often  includes  two  or 
more  categories  of  genotypes.  Thus  F2  dominants  though 
all  may  look  alike  (be  of  one  phenotype)  regularly  include 
both  homozygotes  and  heterozygotes  (wholly  distinct  geno- 
types). 


CHAPTER  X 

CALCULATING  MENDELL^^N  EXPECTATIONS 

Mendelian  expectations  may  be  calculated  either  by  the 
algebraic  method  used  by  Mendel  himself  or  by  the  ingenious 
checkerboard  method  devised  by  Punnett.  The  first  step 
in  either  process  consists  in  ascertaining  what  factorial  com- 
binations are  to  be  expected  among  the  gametes  formed  by 
either  parent.  By  the  algebraic  method,  we  ascertain  the 
product  of  the  gametic  combinations  of  the  two  parents, 
which  will  give  the  zygotic  combinations  to  be  expected 
among  their  Fi  offspring.  A  repetition  of  this  process,  con- 
sidering the  Fi  individuals  now  as  parents,  will  give  the 
combinations  to  be  expected  among  the  F2  offspring,  etc. 

For  example,  if  a  homozygous  colored  guinea-pig  is  crossed 
with  an  albino,  the  gametes  formed  by  the  parents  contain 
C  and  c  respectively.  The  Fi  zygotes  will  contain  the  two  in 
association,  Cc.  The  gametes  formed  by  the  Fi  individuals 
will  contain  either  C  or  c,  or  collectively  will  be  C  -f  c.  The 
Fi  female  will  produce  gametes  (eggs),  C  -|-  c;  the  Fi  male 
will  produce  gametes  (sperms),  C  +  c;  the  F2  zygotes  will 
correspond  with  their  product  or  CC  +  2Cc  -h  cc,  or  one 
homozygous  colored  (CC),  two  heterozygous  colored  (Cc) 
and  one  homozygous  albino  (cc),  or  altogether  three  colored 
to  one  albino,  the  observed  average  result. 

Suppose  now  we  wish  to  calculate  the  result  to  be  expected 
from  a  back-cross  of  Fi  with  the  recessive  (albino)  parent. 
The  Fi  gametes,  we  have  assumed,  are  C  -j-  c;  the  gametes 
of  the  recessive  parent  are  all  c.  Their  product  is  Cc  -|-  cc 
or  equal  numbers  of  heterozygous  colored  individuals  and 
albinos,  the  observed  experimental  result. 

The  checkerboard  method  of  calculating  Mendelian  ex- 
pectations consists  in  writing  the  gametic  contributions  of 
one  parent  in  a  series  of  horizontal  squares,  each  combination 

104 


CALCULATING  MENDELIAN  EXPECTATIONS   105 

in  a  different  horizontal  row.  The  contributions  of  the  other 
parent  are  then  written  in  the  same  squares,  but  in  vertical 
rows,  instead  of  horizontal  ones  (since  their  distribution  con- 
stitutes a  separate  contingency)  each  gametic  combination 
being  entered  in  a  different  vertical  row.  The  checkerboard 
will  then  show  (within  its  individual  squares)  what  factorial 
combinations  are  to  be  expected  among  the  zygotes  (progeny 
of  the  parents  in  question)  and  with  what  frequencies. 

For  the  example  chosen,  the  cross  between  homozygous 
colored  and  albino  guinea-pigs,  all  the  gametes  of  each  parent 

Egg3 


Eggs 

C  c 


01 

a 


a 

C/2 


c 

c 

c 

c  c 

C  c 

c 

c  C 

c  c 

a 


C  c 

c  c 

Fig.  49,     Checkerboard  method  of  cal- 
culating a  Mendelian  F2  expectation. 


Fig.  50.  Checkerboard  method  of  calcu- 
lating the  result  of  a  back-cross  between 
Fi  and  the  recessive  parent. 


being  alike,  the  Fi  zygotes  would  be  all  of  one  sort,  Cc.  But 
since  the  gametes  formed  by  each  Fi  parent  are  of  two  sorts, 
C  and  c,  it  is  evident  that  the  checkerboard  must  contain 
two  horizontal  and  two  vertical  rows,  or  a  total  of  four 
squares.  (See  Fig.  49.)  Let  us  enter  C  in  the  upper  horizon- 
tal row  and  c  in  the  lower  row  as  the  gametic  contributions 
of  one  parent,  then  enter  C  in  the  left  vertical  row  of  squares 
and  c  in  the  right  vertical  row  as  the  contributions  of  the 
other  parent.  We  then  have  the  table  as  shown,  one  square 
containing  CC,  two  containing  Cc,  and  one  cc,  the  same  result 
given  by  the  algebraic  method. 

For  the  back-cross  of  Fi  with  the  recessive  parent,  only 
two  squares  are  required.  (See  Fig.  50.)  The  recessive  parent 
contributes  always  c,  which  we  enter  in  the  two  squares 
placed  in  a  horizontal  row.  The  Fi  parent  contributes  C  to 
one  square,  c  to  the  other.    The  resulting  combinations  are 


106 


GENETICS  AND  EUGENICS 


obviously  Cc  and  cc  respectively.  A  checkerboard  is  scarcely 
necessary  for  cases  as  simple  as  these,  but  will  be  found  very 
clarifying  to  thought  for  the  beginner,  particularly  if  he  is"^ 
not  accustomed  to  thinking  in  algebraic  terms,  when  he  comes 
to  deal  with  crosses  involving  simultaneously  three  or  four 
independent  characters. 

The  essential  point  about  which  one  must  first  of  all  he  entirely 
clear  in  his  own  mind  is  this  —  what  kinds  of  gametes  will  each 
parent  form.  If  he  is  clear  as  to  this  question  the  calculation 
of  expectations  by  either  method  will  present  no  difficulties. 
It  should  be  borne  in  mind  therefore  that  the  fundamental 
Mendelian  assumptions  are  (1)  that  homozygotes  form  only 
one  type  of  gamete  but  (2)  that  heterozygotes  form  two 
types  of  gametes  equally  numerous,  viz.,  dominants  and  re- 
cessives.  Further  (3)  double  heterozygotes  {i.  e.,  individuals 
heterozygous  for  each  of  two  independent  characters)  form 
four  types  of  gametes  all  equally  numerous,  and  (4)  triple 
heterozygotes  form  eight  types  of  gametes,  all  equally  numer- 
ous. (5)  In  general  every  additional  character  in  which  the 
individual  is  heterozygous  doubles  the  assortment  of  gametes 
which  it  would  otherwise  form.    See  Table  7. 


TABLE  7 

Zygotic  Composition  of  Parents  and  the  Expected  Constitution 

OF  THEIR  Gametes 


Parent 
Homozygote,  AA 

AABB 
"  AABBCC 

Heterozygote,  Aa 
Bb 
Cc 
Double  heterozygote,  AaBb 

AaCc 
BbCc 


Triple 


AaBbCc 


Gametes  which  it  will  form 

all  A 
all  AB 
all  ABC 

A  +  a 
B  +  b 

C  +  c 

AB  +  Ab  +  aB  +  ab 

AC  +  Ac  +  aC  +  ac 
BC  +  Be  +  bC  +  be 
ABC  +  ABc  +  AbC  +  aBC 
+  Abe    +  aBc  +  abC  +  abc 


Inspection   of   a  typical   checkerboard    calculation,   that 
for  the  F2  generation  following  a  dihybrid  cross,  shows  some 


CALCULATING  MENDELIAN  EXPECTATIONS  107 

interesting  facts.  All  the  homozygotes  expected  lie  in  the 
diagonal  row  of  squares  running  from  the  upper  left  to  the 
lower  right  comer  of  the  figure.  Compare  Fig.  49.  These 
are  the  individuals  that  will  "breed  true,"  i.  e.,  will  fonn 
only  a  single  type  of  gamete.  They  are  four  in  number,  each 
of  a  different  sort  and  would  result  from  the  union  of  two  like 
gametes  of  each  of  the  four  expected  types,  AB  +  Ab  +  aB  -f 
ab  (or  in  Fig.  49,  EA  +  Ea  +  eA  +  ea).  They  represent  all 
the  possibilities  as  regards  true  breeding  ('*fixed")  forms  to 
be  expected  from  the  cross.  What  the  nature  of  the  other 
individuals  to  be  expected  would  be  would  depend  upon 
the  completeness  of  dominance.  If  dominance  should  be 
complete,  heterozygotes  would  be  indistinguishable  except 
by  breeding  test  from  the  four  expected  homozygotes;  other- 
wise homozygotes  and  heterozygotes  might  be  distinguish- 
able by  appearance  as  well  as  by  breeding  tests.  With 
complete  dominance,  i.  e.,  with  only  dominant  characters 
showing  in  the  zygote,  the  four  sorts  would  appear  as  9  AB : 
3  Ab  :  3  aB  :  1  ab,  the  typical  dihybrid  F2  ratio.  Let  the 
reader  make  out  the  checkerboard  and  verify  these  state- 
ments. 

In  a  similar  way  one  may  calculate,  either  by  algebra  or  by 
checkerboard  the  F2  expected  result  from  a  trihybrid  cross. 
The  eight  kinds  of  gametes  which  the  triply  heterozygous  Fi 
individuals  would  produce  have  already  been  indicated,  viz., 
ABC  -F  ABc  +  AbC  +  aBC  +  Abe  -f  aBc  +  abC  +  abc. 

By  the  checkerboard  method,  each  combination  would 
be  found  homozygous  (united  with  a  gamete  like  itself)  in 
a  different  square  of  the  diagonal  of  the  figure,  and  hetero- 
zygotes containing  the  same  dominant  characters  would  be 
found  elsewhere  in  the  table  sufficient  in  number  to  bring  the 
totals  up  to  27  ABC  :  9  ABc  :  9  AbC  :  9  aBC  :  3  Abc  :  3  aBc: 
3  abC  :  1  abc.  This  is  the  typical  trihybrid  F2  ratio,  when 
complete  dominance  exists. 

To  repeat,  it  is  all  essential  to  determine  first  the  kinds  of 
gametes  each  parent  to  a  mating  is  expected  to  produce.  The 
subsequent  calculation  is  easy  and  certain.    One  soon  learns 


108  GENETICS  AND  EUGENICS 

to  write  out  F2  ratios  without  going  through  the  calculation 
in  detail  either  by  algebra  or  by  checkerboard.  Thus,  if 
we  take  the  expected  completely  recessive  class  as  1,  each 
class  containing  one  dominant  factor  will  be  3,  each  class 
containing  two  dominant  factors  will  be  9  {i,  e.,  3^)  each  class 
containing  three  dominant  factors  will  be  27  {i.  e.,  3^)  etc. 
Accordingly  by  mere  inspection  of  a  gametic  series  to  ascer- 
tain how  many  dominant  factors  each  term  contains,  we  may 
at  once  assign  to  each  the  proportional  number  of  F2  zygotes 
in  which  it  will  be  seen.    See  Table  8. 

TABLE  8 
Relation  between  the  Fi  Gametic  Series  and  the  Expected  F2  Zygotes 

Fi  Gametic  Series  F2  Zygotes 

A  +  a 3A  +  la 

AB  +  Ab  +  aB  +  ab 9  AB  +  3  Ab  +  3  aB  +  1  ab 

ABC  +  ABc  +  AbC  +  aBC\       f  27  ABC  +  9  ABc  +  9  AbC  +  9  aBC 
+  Abe  +  aBc  +  abC  +  abc     J       \  +  3  Abe  +  3  aBc  +  3  abC  +  1  abe 
ABCD  +  ABCd  +  etc.  81  ABCD  +  27  ABCd  +  ete.  (let  the  reader 

supply  the  missing  terms). 

Stated  in  general  terms,  as  Mendel  himself  showed  (and 
as  follows  from  the  binomial  formula),  when  the  number  of 
unit-character  differences  between  the  parents  is  n,  the  visibly 
different  classes  of  offspring  will  be  2",  the  total  different 
sorts  of  zygotes  will  be  3",  and  the  smallest  number  of  in- 
dividuals which  may  be  expected  to  contain  all  of  them  will 
be  4". 

TABLE  9 


Differences 
Between  Parents 

Visibly 
Different  Classes 

Really 
Different  Classes 

Minimum  Number 
of  F2  Individuals 
Including  all  Classes 

n 

271 

3« 

4" 

1 

2 
3 

2 
4 
8 

3 
9 

27 

4 
16 
64 

J 

Tested  by 

Mendel  for 

•    Peas  and 

Found 

Correct 

4 

16 

81 

256^ 

5 

32 

243 

1024 

•    Calculated 

6 

64 

729 

4096 

J 

Table  9  shows  what  the  size  of  these  several  classes  is  for 
1-6  independent  characters. 


CHAPTER  XI 

MODIFIED  MENDELIAN  RATIOS;    HETEROZYGOUS 
CHARACTERS;    ATAVISM  OR  REVERSION 

In  the  last  chapter  MendeHan  ratios  have  been  calculated  on 
the  supposition  that  homozygous  dominants  and  heterozy- 
gous dominants  are  not  distinguishable  from  each  other, 
which  frequently  is  true;  but  if  they  are  distinguishable  from 
each  other,  then  a  larger  number  of  F2  classes  can  be  recog- 
nized and  their  numerical  proportions  are  different.  A  case 
of  this  kind  was  early  recognized  among  plants  by  Correns. 
(See  Fig.  51.)  When  a  white  variety  of  four-o'clock  {Mira- 
hilis)  is  crossed  with  a  red  variety,  Fi  plants  are  produced 
which  bear  pink  flowers,  and  F2  consists  of  whites,  pinks,  and 
reds  in  the  ratio,  1:2:1.  Reds  and  also  whites  breed  true,  but 
pinks  again  produce  the  three  sorts.  This  result  indicates 
that  both  reds  and  whites  are  homozygotes  (RR  and  rr 
respectively)  but  that  pinks  are  regularly  heterozygotes  (Rr) 
and  for  this  reason  do  not  breed  true  but  are  "  unfixable." 
Pink  in  this  case  may  be  called  a  heterozygous  character;  it 
is  for  that  reason  unfixable. 

A  similar  but  even  better-known  case  among  animals  has 
been  described  by  Bateson  and  Punnett,  that  of  the  blue 
Andalusian  fowl.  Birds  of  this  race  are  of  a  slaty  blue  color 
and  are  known  to  fanciers  to  be  unfixable  as  to  color.  When 
blues  are  mated  with  each  other,  chicks  are  obtained  of  three 
distinct  sorts  as  regards  color,  viz.,  blacks,  blues,  and 
"  splashed  whites."  The  blacks  breed  true,  as  also  do  the 
whites,  but  the  blues  invariably  produce  in  every  generation 
the  three  sorts,  of  which  blacks  may  be  called  homozygous 
dominants  (BB),  whites  homozygous  recessives  (bb),  and 
blues  heterozygotes  (Bb) .  But  it  is  clear  that  if  we  so  desig- 
nate them,  dominance  must  be  recognized  to  be  imperfect. 

109 


no 


GENETICS  AND  EUGENICS 


Attempts  of  poultry  men  to  "  fix  "  the  blue  variety  are  mani- 
festly hopeless,  unless  some  new  variation  arises  within  the 
race  which  can  be  secured  in  homozygous  form  and  will  yet 
possess  the  desired  appearance. 

Another  example  of  a  heterozygous  and  so  unfixable  char- 
acter is  found  among  short-horn  cattle.    Here  red  is  a  true- 


/^imbilis  Jalafta 


Fig.  51.  A  diagram  to  show  inheritance  of  flower  color  in  crosses  of  Mirabilis,  the  "four-o'clock." 
Alha,  white  parent;  rosea,  red  parent;  alba  +  rosea,  the  unfixable  Fi  heterozygote,  of  intermediate 
color,  pink.    I.  Gen.  =  Fi.    U.   Gen.  =  F2.     (After  Correns.) 

breeding  type  as  also  is  white,  but  the  heterozygote  between 
red  and  white  is  an  unfixable  roan.    (See  Figs.  62-64.) 

The  effect  which  the  production  of  a  recognizable  hetero- 
zygous form  has  upon  the  typical  r2  monohybrid  ratio 
(3:1)  is  to  convert  it  into  a  1:2:1  ratio,  in  which  each 
parental  type  is  represented  by  one  individual  while  the 
heterozygous  type  is  represented  by  two.     The  typical  di- 


HETEROZYGOUS  CHARACTERS       111 

hybrid  ratio  (9:3:3:1)  we  might  expect  to  see  modified  in  a 
similar  way,  if  a  cross  were  made  involving  simultaneously 
two  Mendelian  characters  imperfectly  dominant.  The  num- 
ber of  distinguishable  classes,  as  shown  originally  by  Mendel 
(see  Appendix)  would  then  be  9,  numerically  as  follows:  1:1: 
2:2:4:2:2:1:1.  For  three  factors  all  imperfectly  dominant 
the  modified  trihybrid  Mendelian  ratio  would  be  expressed 
by  (1  +  2  +  1)  3  and  for  n  factors  by  (1  +2  + 1)'*.  Hetero- 
zygous characters  must  from  definition  always  be  unfixable. 
In  the  foregoing  cases  comparison  of  their  behavior  in  breed- 
ing experiments  with  that  of  the  corresponding  homozygotes 
has  shown  this  to  be  true,  but  there  exist  cases  in  which 
only  one  type  of  homozygote  has  been  found  to  occur,  the 
other  being  apparently  impossible  of  production. 

The  first  case  of  this  sort  to  be  demonstrated  is  found 
among  yellow  mice  and  to  Cuenot  (confirmed  by  Little)  we 
owe  its  demonstration.  If  certain  strains  of  yellow  mice  are 
crossed  with  black  ones,  the  offspring  produced  are  of  two 
sorts  equally  numerous,  yellow  and  black.  From  this  result 
alone  it  is  impossible  to  say  which  is  the  dominant  character, 
but  breeding  tests  of  the  offspring  show  that  yellow  is  the 
dominant  character.  For  the  black  offspring  bred  together 
produce  only  black  offspring,  but  the  yellows  bred  together 
produce  both  yellow  offspring  and  black  ones.  The  curious 
feature  of  the  case  is  that  when  yellows  are  bred  with  each 
other  no  pure  yellows,  that  is,  homozygous  ones,  are  obtained. 
Hundreds  of  yellow  individuals  have  been  tested,  but  the 
invariable  result  has  been  that  they  are  found  to  be  hetero- 
zygous; that  is,  they  transmit  yellow  in  half  their  gametes, 
but  some  other  color  in  the  remaining  gametes  —  it  may  be 
black  or  it  may  be  brown,  or  gray.  Non-yellows  obtained  by 
mating  yellow  with  yellow  mice  never  produce  yellow  off- 
spring if  mated  with  each  other.  This  shows  that  they  are 
genuine  recessives  and  do  not  contain  the  yellow  character, 
which  is  dominant. 

Now  ordinary  heterozygous  dominants,  when  mated  with 
each  other,  produce  three  dominant  individuals  to  one  reces- 


112  GENETICS  AND  EUGENICS 

sive.  Accordingly  we  should  expect  yellow  mice,  if,  as 
stated,  they  are  invariably  heterozygous,  to  produce  three 
yellow  offspring  to  one  of  a  different  color,  but  curiously 
enough  they  do  not.  They  produce  two  yellows  (instead  of 
the  expected  three)  to  every  one  of  a  different  color.  About 
the  ratio  there  can  be  no  reasonable  doubt.  It  has  been 
determined  with  great  accuracy  by  Dr.  C.  C.  Little,  who 
finds  that  in  a  total  of  over  twelve  hundred  young  produced 
by  yellow  parents  almost  exactly  two-thirds  are  yellow. 
Instead  of  the  regular  Mendelian  ratio,  3:1,  we  have  then  in 
this  case  the  peculiar  ratio,  2:1,  and  this  requires  explana- 
tion. The  explanation  of  this  ratio  is  to  be  found  in  the  same 
circumstance  as  is  the  total  absence  of  pure  yellow  individu- 
als. Pure  yellow  zygotes  are  indeed  formed,  but  they 
perish  for  some  reason.  A  yellow  individual  produces  gametes 
of  two  sorts  with  equal  frequency,  viz.,  yellow  and  non- 
yellow  (let  us  say  black).  For,  if  yellow  individuals  are 
mated  w^ith  black  ones,  half  the  offspring  are  black,  half 
yellow,  as  already  stated.  Now  if  yellow  individuals  are 
mated  with  each  other  we  expect  three  sorts  of  young  to  be 
produced,  numerically  as  1:2:1,  viz.,  1  Y  Y,  2  Y  B,  and  1 
B  B.  But  since  observation  shows  that  only  two  combina- 
tions are  formed  which  contain  yellow  to  one  not  containing 
yellow,  and  since  further  all  yellows  which  survive  are  found 
to  be  heterozygous  (YB),  it  must  be  that  the  expected  Y  Y 
individual  either  is  not  produced  or  straightway  perishes. 
As  to  which  of  these  two  contingencies  happens  we  also  have 
experimental  evidence.  Dr.  Little  finds  (confirming  Cuenot), 
that  yellow  mice  when  mated  to  black  ones  produce  larger 
litters  of  young  than  w^hen  they  are  mated  to  yellow  ones. 
The  average-sized  litter  contains  something  like  5.5  young 
when  the  mate  is  a  black  animal,  but  only  4.7  when  it  is  a 
yellow  animal.  It  is  evident,  then,  that  about  one  young  one 
out  of  a  litter  perishes  when  both  parents  are  yellow,  and  this 
undoubtedly  is  the  missing  yellow-yellow  zygote.  The 
yellows  which  are  left  are  heterozygous  yellow-black  zygotes, 
and  they  are  to  those  that  perish  as  2 :1.    They  are  also  to  the 


Fig.  52.  Simple  Mendelian  inheritance  in  crosses  of  red  guinea-pigs  with  black  ones.  P,  parents;  one 
red,  one  black.  Fi,  one  of  the  young,  all  heterozygous  blacks.  BC,  young  produced  by  a  back-cross  of 
an  Fi  black  with  the  red  parent.    Half  are  red,  half  are  black. 


UNFIXABLE  CHARACTERS        113 

non-yellow  zygotes  as  2:1,  the  ratio  observed  also  among  the 
surviving  young  of  yellow  by  yellow  parents. 

This  interpretation  of  the  2:1  ratio  observed  in  this  case  is 
strongly  supported  by  a  similar  case  among  plants,  in  which 
the  evidence  is  even  more  complete.  A  so-called  "  golden  " 
variety  of  snapdragon,  one  in  which  the  foliage  was  yellow 
variegated  with  green,  was  found  by  the  German  botanist, 
Baur,  to  be  unfixable,  producing  when  self-pollinated  fully 
green  plants  as  well  as  golden  ones,  in  the  ratio  2  golden: 
1  green.  The  green  plants  were  found  to  breed  true,  that 
is,  to  be  recessives,  while  the  golden  ones  were  invariably 
found  to  be  heterozygous.  Baur  found,  however,  by  ger- 
minating seeds  of  golden  plants  very  carefully,  that  there 
were  produced  in  addition  to  green  plants  and  golden  ones  a 
few  feeble  seedlings  entirely  yellow,  not  variegated  with 
green,  as  the  golden  plants  are.  These,  for  lack  of  assimilat- 
ing organs  (green  chlorophyl),  straightway  perished.  Clearly 
they  were  the  missing  pure  yellow  zygotes. 

Frequently  one  of  the  visible  characters  of  an  organism 
depends  upon  the  combined  action  of  two  or  more  inde- 
pendent Mendelian  factors,  in  which  case  it  is  demonstrably 
not  a  unit-char SLCter,  as  has  already  been  pointed  out,  since 
each  of  the  known  "  factors  "  is  indispensable  to  the  develop- 
ment of  the  visible  character,  as  are  probably  also  a  great 
many  other  as  yet  unknown  factors.  The  dependence  of  a 
visible  character  upon  two  or  more  simultaneously  varying 
factors  leads  to  the  production  of  modified  dihybrid  or  poly- 
hybrid  F2  ratios.  It  also  leads  to  a  phenomenon  known  as 
atavism  or  reversion,  by  which  is  meant  the  restoration  of  a 
lost  ancestral  character,  which  frequently  follows  crossing  of 
unrelated  varieties. 

Atavism  or  reversion  to  an  ancestral  condition  is  a  phe- 
nomenon to  which  Darwin  repeatedly  called  attention.  He 
realized  that  it  is  a  phenomenon  for  which  general  theories 
of  heredity  must  account.  He  supposed  that  the  environ- 
ment was  chiefly  responsible  for  the  reappearance  in  a  species 
of  a  lost  ancestral  condition,  but  that  in  certain  cases  the 


114  GENETICS  AND  EUGENICS 

mere  act  of  crossing  may  reawaken  slumbering  ancestral 
traits.  Thus  he  noticed  that  when  rabbits  of  various  sorts 
are  turned  loose  in  a  warren  together,  they  tend  to  revert  to 
the  gray-coated  condition  of  wild  rabbits.  And  when  pigeons 
are  crossed  in  captivity  they  frequently  revert  to  the  plum- 
age condition  of  the  wild  rock  pigeon,  Columha  livia.  In 
plants,  too,  Darwin  recognized  that  crossing  is  a  frequent 
cause  of  reversion.  The  explanation  which  he  gave  was  the 
best  that  the  knowledge  of  his  time  afforded,  but  it  leaves 
much  to  be  desired.  This  lack,  however,  has  been  completely 
supplied  by  the  Mendelian  principles.  An  illustration  or  two 
may  now  be  cited. 

When  pure-bred  black  guinea-pigs  are  mated  with  red  ones, 
only  black  offspring  are  as  a  rule  obtained.  (See  Fig.  52.) 
The  hairs  of  the  offspring  do  indeed  contain  some  red  pig- 
ment, but  the  black  pigment  is  so  much  darker  that  it  largely 
obscures  the  red.  In  other  words,  black  behaves  as  an  ordi- 
nary Mendelian  dominant.  In  the  next  generation  black  and 
red  segregate  in  ordinary  Mendelian  fashion,  and  the  young 
produced  are  in  the  usual  proportions,  three  black  to  one  red, 
or  1:1  in  back-crosses  of  the  heterozygous  black  with  red. 
All  black  races  behave  alike  in  crosses  with  the  same  red 
individual,  but  among  red  animals  individual  differences 
exist.  Some,  instead  of  behaving  like  Mendelian  recessives, 
produce  in  crosses  with  a  black  race  a  third  apparently  new 
condition,  but  in  reality  a  very  old  one,  the  agouti  type  of 
coat  found  in  all  wild  guinea-pigs,  as  well  as  in  wild  rats,  mice, 
squirrels,  and  other  rodents.  In  this  type  of  coat  reddish 
yellow  pigment  alone  is  found  in  a  conspicuous  band  near  the 
tip  of  each  hair,  while  the  rest  of  the  hair  bears  black  pig- 
ment. The  result  is  a  brownish  or  grayish  ticked  or  grizzled 
coat,  inconspicuous,  and  hence  protective  in  many  natural 
situations.     (See  Fig.  53.) 

Some  red  individuals  produce  the  reversion  in  half  of  their 
young  by  black  mates,  some  in  all,  and  others,  as  we  have 
seen,  in  none,  this  last  condition  being  the  commonest  of  the 
three.    It  is  evident  that  the  reversion  is  due  to  the  intro- 


Fig.  53.  Reversion  in  crosses  of  a  red  guinea-pig  with  a  black  one.  P,  parents.  Fi,  one  of  the  rever- 
sionary (agouti)  young.  BC,  young  produced  by  a  back-cross  of  an  Fi  agouti  with  an  ordinary  red 
individual.    Half  the  young  are  red.    The  other  half  are  equally  divided  between  agoutis  and  blacka. 


REVERSION  OR  ATAVISM  115 

duction  of  a  new  factor,  additional  to  simple  red  or  simple 
black.  It  is  evident  further  that  this  new  factor,  which  we 
will  call  A  (agouti),  has  been  introduced  through  the  red 
parent,  and  that  as  regards  this  factor,  A,  some  red  indi- 
viduals are  homozygous  (AA)  in  character,  others  are  hetero- 
zygous (Aa),  while  others  lack  it  altogether  (aa).  The  agouti 
character  becomes  visible  only  in  the  presence  of  both  black 
and  red,  because  it  is  a  mosaic  of  those  two  pigments.  If  the 
Fi  agouti  individuals  are  bred  together  they  produce  in  the 
next  generation  (F2)  three  sorts  of  young,  viz.,  agouti, 
black,  and  red,  which  are  numerically  as  9:3:4.  This  evi- 
dently is  a  modification  of  the  dihybrid  Mendelian  ratio 
9:3:3:1,  resulting  from  the  fact  that  the  last  two  classes  are 
superficially  alike.  They  are  red  animals  with  and  without 
the  agouti  factor  respectively;  but  this  agouti  factor  is  in- 
visible in  the  absence  of  black,  so  that  both  sorts  of  reds  look 
alike.  Together  they  number  four  in  sixteen  of  the  F2  off- 
spring. Figure  54  is  intended  to  show  by  the  checkerboard 
method  how  this  modified  dihybrid  ratio  is  obtained. 

Black  and  red  varieties  differ  from  each  other  by  a  varia- 
tion in  what  has  been  called  the  extension  factor  (E),  the 
reference  being  to  the  fact  that  black  (or  brown)  pigment, 
found  in  the  eyes  of  both  varieties,  extends  throughout  the 
coat  in  the  black  variety  but  is  restricted  to  the  eye  in  the 
red  variety.  The  allelomorphic  conditions  of  this  factor  are 
designated  E  (in  black)  and  e  (in  red)  respectively.  The 
agouti  factor  (A)  may  exist  in  red  animals  without  producing 
visible  effects  because  there  is  no  black  pigment  in  the  fur 
of  such  animals  to  bring  out  the  ticking,  but  its  existence  in 
animals  which  would  otherwise  be  black  changes  the  coat  to 
agouti.  Hence  the  constitution  of  the  parental  gametes  is: 
Black  parent,  Ea;  red  parent  eA.  Fi  is  EeAa,  a  double 
heterozygote.  Its  gametes  are  EA  +  Ea  +  eA  +  ea,  which 
with  dominance  complete  will  produce  F2  zygotes,  9  EA  + 
3  Ea  -1-  3  eA  -h  1  ea.  (See  Figure  54.)  But  EA  contains  the 
two  factors  which  together  produce  agouti;  Ea  contains  the 
factors  for  black;  eA  contains  the  factor  for  agouti  but  with- 


116 


GENETICS  AND  EUGENICS 


out  the  factor  (E)  necessary  to  make  it  visible,  and  so  will  be 
red;  and  ea  contains  neither  the  factor  for  agouti  nor  that 
for  black,  hence  will  also  be  red.  Accordingly  the  expected 
F2  distribution  is  nine  agouti,  three  black,  four  red,  the  ratio 
observed.     This  is  a  very  common  modification  of  the  F2 


E  A 


E  a 


e  A 


e  a 


E  A 


E  a 


e  A 


e  a 


E  A 

E  A 

E  A 

EA 

E  A 

E  a 

e  A 

e  a 

Agouti 

Agouti 

Agouti 

Agouti 

E  a 

E  a 

E  a 

E  a 

EA 

E  a 

e  A 

e  a 

Agouti 

Black 

Agouti 

Black 

e  A 

e  A 

e  A 

e  A 

EA 

E  a 

e  A 

e  a 

Agouti 

Agouti 

Red 

Red 

e  a 

e  a 

e  a 

e  a 

E  A 

E  a 

eA 

e  a 

Agouti 

Black 

Red 

Red 

Fig.  54.  Checkerboard  to  explain  the  modified  dihybrid  F2  ratio, 
9  :  3  :  4,  as  observed  when  black  guinea-pigs  are  crossed  with  red  ones 
which  transmit  the  agouti  factor  (A). 

dihybrid  ratio  and  owes  its  production  to  the  fact  that  two 
independent  Mendelian  factors  are  involved  one  of  which  pro- 
duces no  visible  effect  except  in  the  presence  of  the  other. 

Another  example  of  this  same  modified  dihybrid  ratio 
(9:3:4)  is  obtained  by  crossing  an  albino  rodent  (rat,  mouse, 
rabbit  or  guinea-pig)  derived  from  a  black  race,  with  a  wild 
(agouti)  individual.  Fi  consists  of  agoutis,  like  the  wild 
parent,  but  F2  contains  agoutis,  blacks,  and  albinos  in  the 
proportions,  nine  agouti,  three  black,  four  albino.  The  ex- 
planation is  as  follows.  The  albino  parent  differs  from  the 
wild  agouti  parent  as  regards  two  factors,  viz.,  the  color 
factor  (C)  and  the  agouti  factor  (A).    The  albino  parent  is 


% 


Fig.  5.5.  Reversion  to  full  intensity  of  pigmentation  on  crossing  a  pink-eyed  cream-and-white  rat  with 
an  albino.  P,  parents;  cream-and-white  at  left,  albino  at  right.  Fi,  one  of  the  black-and-white  young. 
F2,  cream-and-white  at  left,  black-and-white  in  middle,  albino  at  right.  Their  numerical  relations  are 
about  as  3  : 9 : 4.  A  slight  departure  from  these  proportions  is  observed  on  account  of  linkage  (in  this 
case  repulsion)  between  the  genes  for  pink-eye  and  albinism.   See  chapter  on  Linkage. 


i 


MODIFIED  F2  RATIOS  117 

ac;  the  agouti  parent  AC.  Fi  is  AaCc,  a  double  hetero- 
zygote.  Its  gametes  consequently  should  be  of  four  types, 
viz.,  AC  +  Ac  +  aC  +  ac,  and  the  F2  zygotes,  9  AC:3  Ac: 
3  aC  :1  ac.  But  only  zygotes  which  contain  C  will  develop 
a  colored  coat,  hence  both  3  Ac  and  1  ac  will  be  albinos.  The 
9  AC  individuals  contain  the  factors  of  the  wild  parent  and 
hence  will  be  agouti;  the  3  aC  individuals  will  develop  a 
colored  coat  since  they  contain  C,  but  this  coat  will  be  non- 
agouti  (a),  i.  e.,  they  will  be  like  the  wild  type  except  for  the 
lack  of  the  agouti  factor  and  so  will  be  black. 

Precisely  the  same  result  in  Fi  and  F2  is  obtained  if  a  black 
rodent  (rat,  mouse,  rabbit,  or  guinea-pig)  is  crossed  with  an 
albino  which  transmits  the  agouti  factor,  as  for  example  an 
albino  whose  parents  were  homozygous  for  the  agouti  factor. 
In  this  case  Fi  is  agouti  by  reversion,  C  being  derived  from 
the  black  parent,  A  from  the  albino  parent.  But  Fi  is  doubly 
heterozygous,  precisely  as  in  the  foregoing  case,  and  the  F2 
generation  contains  only  three  apparent  classes  of  individuals 
instead  of  the  usual  four  for  the  reason  that  one  of  the  two 
differential  factors  concerned  in  the  cross  (viz..  A)  is  unable 
to  produce  a  visible  effect  except  in  the  presence  of  the 
other  (C). 

Another  somewhat  similar  case  involving  reversion  in  Fi 
with  the  production  of  the  modified  dihybrid  ratio,  9:3:4, 
in  F2  is  illustrated  in  Fig.  55.  A  pale-coated  "  cream-and- 
white  "  rat  was  crossed  with  an  albino  and  produced  black- 
and-white  young,  a  reversion  to  pigmentation  of  full  inten- 
sity, though  white  spotting  was  retained,  this  being  an 
independent  Mendelian  character  transmitted  by  both 
parents.  The  F2  generation  consisted  of  black-and-white, 
cream-and- white,  and  albino  individuals  in  numbers  approxi- 
mating the  9:3:4  ratio.  Black-and-white  is  here  the  double 
dominant  class,  9;  cream-and-white  is  the  single  dominant 
class,  3;  and  the  albinos  include  three  which  transmit  the 
dominant  character,  black-and-white,  but  which  fail  to  show 
it  because  they  lack  the  color  factor,  and  also  one  which 
transmits  cream-and-white  but  which  fails  to  show  it  for  the 


118  GENETICS  AND  EUGENICS 

same  reason,  lack  of  the  color  factor.  Together  the  albinos 
number  four. 

A  different  modification  of  the  typical  dihybrid  ratio  is 
illustrated  by  the  following  case  in  which  two  varieties  were 
crossed  which  possessed  complementary  factors  neither  of 
which  is  able  to  produce  a  visible  effect  apart  from  the  other. 
When  certain  white-flowered  varieties  of  sweet  peas  are 
crossed  with  each  other  they  produce  Fi  plants  which  bear 
red-colored  flowers  (Bateson  and  Punnett).  F2  consists  of  two 
apparent  varieties  only,  viz.,  reds  and  w^hites  in  the  ratio, 
nine  red  to  seven  w^hite.  This  is  explained  as  a  modified  di- 
hybrid ratio  (9:3:3:1)  in  which  the  last  three  terms  are  in- 
distinguishable (all  being  white).  The  two  factors  involved 
in  this  case  are  assumed  to  be  a  color  factor  found  in  one  white 
parent  and  a  red  factor  found  in  the  other,  both  together  (in 
Fi)  producing  a  red  color,  but  either  by  itself  producing  no 
color  whatever.  One  parent  accordingly  produces  gametes 
all  Cr,  the  other  produces  gametes  all  cR.  Fi  is  CcRr,  a 
double  heterozygote;  its  gametes,  CR  -f  Cr  +  cR  -\-  cr; 
and  the  F2  zygotes  containing  the  same  assortments  of  fac- 
tors are  9CR:3Cr:3cR  :lcr.  But  if  C  and  R,  neither  of 
them,  produce  color  apart  from  each  other,  then  only  the 
9  CR  zygotes  are  colored,  all  the  others,  seven  in  sixteen, 
being  white,  and  the  observed  F2  ratio  (9 : 7)  is  thus  accounted 
for  as  the  result  of  a  dihybrid  cross  at  the  same  time  that  the 
Fi  result  is  explained. 

When  some  other  white-flowered  varieties  of  sweet  peas 
are  crossed  with  each  other,  there  are  produced,  not  red- 
flowered  Fi  plants  as  in  the  foregoing  case,  but  those  which 
are  purple  bi-color,  like  the  wild  sweet  pea,  a  case  of  rever- 
sion or  atavism,  like  those  known  for  pigeons,  rabbits  and 
guinea-pigs.  This  reversion  involves  a  third  independent 
factor  (a  factor  for  blue,  B)  which  is  ineffective  except  in 
the  presence  of  both  the  color  factor  (C)  and  the  red  factor 
(R).  When  in  such  reversionary  crosses  a  colored  Fi  is  pro- 
duced which  is  heterozygous  for  all  three  factors,  F2  mani- 
fests a  peculiar  modified  trihybrid  ratio,  less  common  than 


MODIFIED  F2  RATIOS  119 

the  modified  dihybrid  ratios  just  discussed.  If,  for  example, 
one  white  parent  contributes  the  color  factor  while  the  other 
parent  contributes  the  red  and  the  blue  factors,  then  we  may 
represent  the  parental  gametes  as  Crb  and  cRB  respectively. 
Fi  will  then  be  a  triple  heterozygote,  CcRrBb,  which  from 
the  combined  action  of  the  three  dominant  characters  will  be 
a  purple  bi-color.  Its  gametes  will  then  be  of  eight  sorts  and 
the  zygotes  in  which  corresponding  groupings  of  the  domi- 
nant factors  occur  will  be  as  follows:  ^ 

27  CRB,  purple  3  Crb,  white 

9  CRb,  red  3  cRb,  white 

9  CrB,   white  3  crB,  white 

9  cRB,  white  1  crb,  white 

But  only  the  first  two  of  these  eight  groupings  contain  com- 
binations of  factors  capable  of  producing  colored  flowers,  viz., 
CRB,  which  will  produce  purples,  and  CRb,  which  will  pro- 
duce reds.  All  the  other  six  combinations  lack  one  or  both 
of  the  two  factors  (C  and  R)  which  must  be  present  together 
in  order  to  produce  colored  flowers.  Consequently  all  will 
produce  uncolored  (white)  flowers,  and  the  expected  classes 
of  phenotypes  will  be  as  follows:  twenty-seven  purple,  nine 
red,  twenty-eight  white,  a  modified  trihybrid  ratio. 

Summary  on  Modified  Ratios 

1.  When  a  cross  involves  two  factors,  one  of  which  pro- 
duces no  visible  effect  except  in  the  presence  of  the  other, 
the  dihybrid  r2  ratio,  9:3:3:1,  is  modified  to  9:3:4,  because 
the  last  two  classes  of  the  typical  ratio  are  indistinguishable. 

2.  When  a  cross  involves  two  factors,  neither  of  which  pro- 
duces a  visible  result  in  the  absence  of  the  other,  the  dihybrid 
ratio  becomes  9:7,  because  the  last  three  classes  of  the 
typical  ratio  are  indistinguishable;  if  in  addition  a  third 
factor  is  involved  which  produces  no  visible  effect  except  in 
the  presence  of  both  the  others,  a  modified  trihybrid  ratio 
is  obtained,  viz.,  27:9:28. 

^  It  is  suggested  that  the  reader  make  out  the  trihybrid  checkerboard  calcula- 
tion for  this  cross  and  color  the  squares  with  crayon  in  accordance  with  the  assump- 
tion made. 


120 


GENETICS  AND  EUGENICS 


Modification  of  the  ratio,  9:3:3:1,  due  to  linkage.  When  two  Mendelian 
characters  are  not  wholly  independent  of  each  other,  but  show  a  tendency 
to  be  inherited  together,  they  are  said  to  be  coupled  or  linked  to  each  other. 
Thus,  in  the  sweet  pea,  purple  and  red  are  alternative  color  forms,  and 
long  pollen  and  short  pollen  are  alternatives  as  to  pollen  shape.  And  if  a 
purple  plant  with  long  pollen  is  crossed  with  a  red  plant  having  round 
pollen,  four  classes  are  obtained  in  r2,  viz.,  purple  long,  purple  round, 
red  long  and  red  round.     This  being  apparently  a  dihybrid  Mendelian 

TABLE   10 

The  F2  Ratio,  9:3:3:1,  as  Affected  by  Coupling  or  Linkage,  A  and  B 
Entering  the  Fi  Zygote  in  the  Same  Gamete 


Ratio,  Crossover  to 

Proportion 
Crossover 
Gametes 

F2  Zygotes 

Non-crossover 
Gametes 

AB 

Ab 

aB 

ab 

Total 

\:x 

1 

3x2  _|_  2(2a:  -j-  1) 

2x+  1 

2x+  1 

X2 

(2x  +  2)  2 

x-\-  1 

1:11 

1/2 

9 

3 

3 

1 

16 

1:2 

1/3 

22 

5 

5 

4 

36 

1:3 

1/4 

41 

7 

7 

9 

64 

1:4 

1/5 

66 

9 

9 

16 

100 

1:5 

1/6 

97 

11 

11 

25 

144 

1:6 

1/7 

134 

13 

13 

36 

196 

1:7 

1/8 

177 

15 

15 

49 

256 

1:8 

1/9 

226 

17 

17 

64 

324 

1:9 

1/10 

281 

19 

19 

81 

400 

1:99 

1/100 

29,801 

199 

199 

9,801 

40,000 

Limiting  values^ 

.... 

3 

0 

0 

1 

4 

1  No  coupling. 

2  Not  distinguishable  from  the  case  in  which  A  and  B  are  due  to  a  single  genetic  factor. 


cross,  we  should  expect  the  four  classes  to  be  respectively  as  9:3:3:1,  but 
in  reality  the  classes  purple  long  and  red  round  (the  parental  combinations) 
are  in  excess  of  these  proportions.  When  these  facts  were  discovered  by 
Bateson  and  Punnett,  it  was  stated  that  coupling  exists  between  the 
characters  purple  and  long  and  their  allelomorphs  red  and  round.  Later, 
however,  when  a  cross  was  made  between  purple  round  and  red  long,  it 
was  found  that  these  combinations  were  in  excess  in  r2.  Purple  and  long 
which  in  the  first  case  were  coupled,  now  showed  repulsion.  Morgan 
explains  both  cases  by  supposing  that  the  two  character-pairs  have  deter- 
miners or  genes  located  near  to  each  other  in  the  germ-cell,  probably  in  the 
same  chromosome,  so  that  the  parental  combination  has  a  tendency  to 
persist  in  F2.  Morgan  also  proposes  to  substitute  a  single  term,  linkage, 
for  the  two  terms  of  Bateson,  coupling  and  repulsion. 


F2  RATIOS  MODIFIED  BY  LINKAGE 


121 


It  is  evident  that  linkage  will  cause  modification  of  the  tj'pical  dihybrid 
ratio,  9:3:3:1,  since  the  four  possible  classes  of  gametes  formed  by  Fi 
individuals  will  not  all  be  equally  numerous.  Accordingly  the  stronger 
the  linkage,  the  greater  will  be  the  modification  of  the  tj^pical  ratio.  Con- 
versely, we  may  estimate  the  strength  oj  the  linkage  by  the  observed  depart- 
ure from  the  9:3:3:1  ratio. 

In  so  doing,  tables  10  and  11  may  be  found  useful,  in  which  the  expected 
modification  of  the  9:3:3:1  F2  ratio  is  given  for  various  integral  ratios  of 

TABLE   11 

The  F2  Ratio,  9:3:3:1,  as  Affected  by  Repulsion  (Negative  Linkage), 
A  AND  B  Entering  the  Fi  Zygote  in  Different  Gametes 


Ratio,  Crossover  to 

XT 

Proportion 
Crossover 
Gametes 

F2  Zygotes 

Non-crossover 
Gametes 

AB 

Ab 

aB 

ab 

Total 

l:x 

1 

2(x2  4-  2x)  4-  3 

x2+2x 

x2+2x 

1 

(2x+2)2 

x+1 

1:13 

1/2 

9 

3 

3 

16 

1:2 

1/3 

19 

8 

8 

36 

1:3 

1/4 

33 

15 

15 

64 

1:4 

1/5 

51 

24 

24 

100 

1:5 

1/6 

73 

35 

35 

144 

1:6 

1/7 

99 

48 

48 

196 

1:7 

1/8 

129 

63 

63 

256 

1:8 

1/9 

163 

80 

80 

324 

1:9 

1/10 

201 

99 

99 

400 

1:99 

1/100 

20,001 

9,999 

9,999 

40,000 

Limiting  values  * 

.... 

2 

1 

1 

0 

4 

3  No  repulsion. 

*  Not  distinguishable  from  the  case  in  which  A  and  B  are  allelomorphs. 


gametes  showing  the  'parental  combinations,  to  gametes  not  showing  them. 
Morgan  calls  the  gametes  which  show  novel  combinations  crossover 
gametes  and  those  which  show  the  original  combinations  non-crossover 
gametes.  If  the  latter  are  two,  three,  four,  etc.,  times  as  numerous  as  the 
former,  then  we  get  the  modified  F2  ratios  shown  in  the  tables,  where  also 
formulae  are  given  for  extending  the  tables  to  any  desired  extent.  In 
making  use  of  these  tables,  it  is  necessary  only  to  reduce  to  the  basis  of  a 
common  total  the  observed  F2  zygotic  series  and  any  series  of  the  table 
with  which  a  comparison  is  desired.  This  will  be  facilitated  by  consulting 
Table  11a,  in  which  each  zygotic  class  of  Tables  10  and  11  is  expressed  as 
a  percentage  of  the  total  population. 


122 


GENETICS  AND  EUGENICS 


TABLE  11a 

A  Combination  of  Tables  10  and  11,  in  which  the  Size  of  each 
F2  Class  is  expressed  as  a  Percentage  of  the  F2  Population. 
It  is  assumed  that  the  Gametic  Series  is  the  same  in  both  Sexes 


Percentage  F 

'i  zygotes  when 

Ratio,  cross- 
over to  non- 
crossover 
gametes 

A  and  B  enter  together 
(Table  10) 

A  and  B  enter  separately 
(Table  11) 

AB 

Ab  (or  aB) 

ab 

AB 

Ab  (or  aB) 

ab 

1 

56.2 

18.7 

6.2 

56.2 

18.7 

6.2 

2 

61.1 

13.6 

11.1 

52.8 

22.2 

2.8 

3 

64.0 

10.9 

14.0 

51.5 

23.4 

1.5 

4 

66.0 

9.0 

16.0 

51.0 

24.0 

1.0 

5 

67.3 

7.6 

17.3 

50.7 

24.3 

0.7 

6 

68.3 

6.6 

18.3 

50.5 

24.5 

0.5 

7 

69.3 

5.8 

19.3 

50.4 

24.6 

0.4 

8 

69.7 

5.2 

19.7 

50.3 

24.7 

0.3 

9 

70.2 

4.7 

20.2 

50.2 

24.8 

0.2 

99 

74.5 

0.5 

24.5 

50.0+ 

24.9 

0.0+ 

oc 

75.0 

0.0 

25.0 

50.0 

25.0 

0.0 

In  using  Table  11a  to  test  a  case  of  suspected  linkage,  the  size  in  per  cent 
of  each  observed  F2  class  should  first  be  determined  and  comparison  made 
with  the  corresponding  class  in  the  Table.  In  cases  of  doubt,  determina- 
tion of  the  probable  error  of  linkage  may  show  whether  the  observed  de- 
partures from  the  normal  9:3:3:1  ratio  are  or  are  not  significant.  The 
9:3:4  ratio  as  affected  by  linkage  may  be  obtained  by  combining  in  Tables 
10  or  11,  the  numbers  in  the  columns  headed  aB  and  ab. 


Fig.  56.  A  dihybrid  Mendelian  cross  between  a  wild  Norway  rat  and  the  tame  variety  known  as 
black  hooded.  P,  parents;  wild  gray  at  left,  black  hooded  at  right.  Fi,  a  heterozygote,  gray  like  the 
wild  parent,  but  showing  traces  of  the  recessive  white  spotting.  Note  white  left  fore  foot.  F;,  the  four 
second-generation  classes  of  offspring.  From  left  to  right,  gray  self,  gray  hooded,  black  self,  black 
hooded.    Numerically  as  9  :  3  :  3  : 1.    Let  the  reader  identify  in  Table  12  the  unit-characters  involved. 


« 


CHAPTER  XII 

THE  UNIT-CHARACTERS  OF  RODENTS 

No  group  of  mammals  has  been  studied  as  thoroughly,  in 
respect  to  heritable  characters,  as  have  the  rodents.  This  is 
particularly  true  as  regards  those  striking  variations  of  the 
coat  which  form  the  basis  of  the  many  recognized  domestic 
varieties.  In  nearly  every  case  the  distinctive  features  of 
these  several  varieties  are  found  to  be  Mendelian  unit-char- 
acters. As  an  example  we  may  take  the  varieties  of  the 
domestic  cavy  or  guinea-pig,  probably  the  first  of  the  rodents 
in  point  of  time  to  be  domesticated.  Certainly  in  richness  of 
varieties  it  surpasses  all  others.  It  was  domesticated  by  the 
ancient  Peruvians  before  the  discovery  of  America  and 
formerly  held  an  important  economic  place  among  the  natives 
of  tropical  America  where  it  was  reared  as  an  article  of  food 
in  every  cabin,  a  practice  which  to  some  extent  still  continues 
among  the  poorer  classes.  Its  variation  in  color  and  other 
coat  characters  has  been  very  extensive,  unequalled  in 
amount  perhaps  among  mammals  other  than  dogs.  Nearly 
every  distinct  variety  is  characterized  by  the  possession  of 
one  or  more  Mendelian  unit-character  variations.  At  least 
ten  such  unit-characters  are  concerned  in  the  production  of 
these  varieties.  Several  of  these  unit-characters  have  al- 
ready been  referred  to.  (See  Table  12.)  All  but  one  of  them 
("  rough  ")  may  be  regarded  as  recessive  unit-character  va- 
riations from  the  conditions  found  in  wild  cavies  generally. 
Perhaps  the  earliest  in  point  of  time,  certainly  the  com- 
monest among  rodents  wild  or  domesticated,  is  the  albino 
variation,  in  which  the  fur  is  white  and  the  eye  pink.  This 
makes  its  appearance  as  a  sport,  probably  originally  in  a 
single  individual  and  later  as  a  recurring  variation  among  its 
descendants.  Albino  individuals  are  undoubtedly  at  a  dis- 
advantage in  the  struggle  for  existence  in  a  wild  state  because 
of  the  conspicuousness  of  the  albino  to  its  enemies  and  also 

122a 


UNIT-CHARACTERS  OF  RODENTS 


123 


because  of  its  defective  vision.  For  the  eyesight  of  the  albino 
is  very  poor  owing  to  the  imperfect  pigmentation  of  its  eyes. 
Albino  sports  accordingly  never  become  very  common  in  a 
wild  species  but  are  probably  among  the  earliest  formed 
domestic  or  tame  varieties,  because  of  their  striking  character 

Table  12 
Some  Unit-Characters  of  Rodents 


Name  of  Factor 

Symbol, 
Dominant 

Appearance  of  Dominant 

Symbol, 
Recessive 

Appearance  of  Recessive 

Phase 

Individual 

Phase 

Individual 

Color 

c 

Colored 

c 

Albino 

Extension 

E 

Black  or  brown 

e,e' 

Yellow,  yellow  spotted 
with  black  or  brown 

Agouti 

A 

Gray  (agouti) 

a 

Black  or  brown  (non- 
agouti) 

Black 

B 

Black  or  black  agouti 

b 

Brown  or  brown  agouti 

WTiite  spotting 

S 

Self  colored 

s 

Spotted  with  white 

Dark  eye 

D 

Dark  eyes  and  coat 

d 

Pink  eyes  and  coat 
pale,  where  not  yel- 
low 

Intensity 

I 

All  pigments  dark 

i 

All  pigments  pale 

Hair  length 

L 

Short-haired  like  wild 
cavies 

1 

Hair  long  and  silky 

Rough  coat 

R 

Coat  rosetted 

r 

Coat  smooth 

Rough  modi- 

M 

Coat  slightly  rosetted 

m 

Coat  rosettes  fully  de- 

fier 

veloped 

and  the  ease  with  which  a  distinct  variety  is  established. 
For,  being  recessive,  the  albino  variation  is  secure  as  a  racial 
character  as  soon  as  a  pair  of  albinos  has  been  isolated.^ 

The  albino  variation  is  commonly  considered  to  be  the  re- 
sult of  a  recessive  variation  in  a  color  factor  whose  dominant 
phase  is  expressed  by  the  symbol,  C,  its  recessive  or  albino 
phase  by  c.     (See  Table  12.) 

Another  color  sport  occasionally  observed  among  wild  ro- 
dents, and  which  is  the  basis  of  distinct  varieties  among 

^  The  contemporary  origin  of  an  albino  race  of  field  mouse  (Peromyscus)  has 
recently  been  recorded  (Castle,  1912)  in  a  species  in  which  neither  this  nor  any  other 
of  the  common  color  sports  had  previously  been  recorded. 


124  GENETICS  AND  EUGENICS 

tame  ones,  is  a  change  to  yellow  coat.  This  results  from  a 
disappearance  of  black  pigment  from  the  hair  or  its  replace- 
ment by  yellow.  But  the  black  pigment  still  persists  in  the 
eye.  Hence  one  ma\^  speak  of  this  change  as  being  a  restric- 
tion of  black  to  the  eye,  whereas  in  wild  rodents  it  is  regu- 
larly extended  throughout  the  coat.  The  factor  which  has 
undergone  change  is  therefore  said  to  be  the  extension  factor 
for  black  (or  brown)  pigment.  Its  dominant  phase  may  be 
expressed  by  E,  its  recessive  phase  (found  in  yellow  animals) 
by  e.  (See  Plate  7,  Fig.  29.)  An  alternative  recessive  phase 
(eO  is  found  in  yellow  animals  spotted  or  brindled  with  black 
or  brown. ^ 

A  third  sport  among  wild  rodents  is  responsible  for  the 
origin  of  black  varieties  which  lack  the  yellow  tip  of  the  fur 
found  in  most  wild  gray  or  ''agouti "  varieties.  (See  Plates  6 
and  7,  Figs.  22-26).  This  yellow  tip  sometimes  takes  the 
form  of  a  subapical  band  of  yellow  on  hair  which  is  black  (or 
brown)  both  at  the  base  and  at  the  extreme  end.  This  is  the 
case  for  example  in  the  agouti  varieties  of  the  rabbit  and  the 
guinea-pig.  The  optical  effect  of  the  agouti  factor  in  either 
case  is  to  produce  a  protectively  colored,  neutral  gray  coat, 
inconspicuous  against  many  natural  backgrounds.  The  black 
sport  may  be  regarded  as  a  recessive  variation  in  an  agouti 
factor  possessed  by  most  wild  rodents.  The  dominant  phase 
of  this  factor  may  be  expressed  by  A,  its  recessive  phase  (the 
non-agouti  variation)  by  a.^ 

Another  unit-character  variation  found  in  many  rodents, 
as  well  as  in  some  other  mammals,  is  responsible  for  the  re- 
placement of  black  pigment  by  brown  throughout  the  coat 
and  even  in  the  eye.    (See  Plate  7,  Figs.  27  and  28.)    This 

^  The  occurrence  of  yellow  sports  among  wild  meadow  mice  (Microtus)  has  been 
observed  by  Cole,  Barrows,  F.  Smith  and  others,  though  no  tame  races  of  this  very 
common  rodent  have  yet  been  established.  The  contemporary  origin  in  England 
of  a  yellow  race  of  the  Norway  rat  has  been  recorded  by  Castle  (1914),  and  the 
origin  of  a  yellow  race  of  Mus  rattus  by  Bonhote. 

2  Sometimes  black  varieties  arise  by  a  process  other  than  a  change  in  the  agouti 
factor,  as  is  the  case  probably  in  a  locally  common  black  variety  of  the  gray  squirrel 
of  Eastern  North  America.  This  shows  the  agouti  marking  of  the  fur  to  so  small  an 


Plates  6  and  7  are  reproduced  by  permission  from  Publication 
No.  2Ji^l  of  the  Carnegie  Institution  without  change  of  figure  num- 
bers. They  show  in  the  natural  colors  how  a  single  pure-breeding 
domestic  type  {20)  crossed  with  a  single  pure-breeding  wild  type 
{23  and  2Jt)  may  produce  in  the  next  generation  only  a  single  type 
{22)  y  which  however  may,  in  the  following  generation,  through  the 
operation  of  MendeVs  law,  produce  half-a-dozen  very  distinct  pure- 
breeding  types  {25-30) .  Through  a  knowledge  of  MendeVs  law 
the  multiplication  of  color  types  among  animals  and  plants  has 
ceased  to  be  a  haphazard  process  and  has  become  a  simple  and 
orderly  procedure. 


-J, 


0 


^« 


"■^s^S 


"ig.   20,   half-grown  guinea-pig,   race  C.       Figs.   23,   24,    male  and  female    Cavia  cutleri,   adult 
Fig 

Fip-.     34^   X    Cavia    riitleri.    adult 


22,   Fi  hybrid,   race  C  x  Cavia   cutleri,   adult.       Fig.    21,    Fi  hybrid,   race  B  (Plate  5 


%.aK  oA/eiff'ii'dliifSuj.gXi.    affatoiir^ 


F2  hybrids,   race  Cx  Cavia  cutleri.     Fig.  25,  agouti;    26,  black;    27,  chocolate;    28,  cinnamon; 
29,  yellow;    30,  albino. 


i 


UNIT-CHARACTERS  OF  RODENTS  125 

change  converts  an  ordinary  gray  variety  into  a  ** cinna- 
mon"  variety,  and  black  into  "chocolate,"  while  yellow  with 
black  eyes  becomes  changed  to  yellow  with  brown  eyes.  The 
factor  which  in  such  cases  has  undergone  change  we  may 
call  the  black  factor,  its  original  or  dominant  phase  being 
expressed  by  B,  the  recessive  (brown)  phase  by  b.  (See 
Table  12.) 

Another  unit-character  color  variation  perhaps  commoner 
than  any  of  those  yet  mentioned  is  found  both  among  wild 
and  among  domesticated  mammals.  It  consists  in  spotting 
with  white.  It  takes  the  form  among  wild  rodents  of  a  white 
spot  in  the  forehead  (common  among  wild  rabbits)  or  a  white 
spot  on  the  belly,  a  white  foot,  or  a  white-tipped  tail.  Rarely 
does  it  go  beyond  these  slight  and  inconspicuous  markings, 
probably  for  the  reason  that  it  would  render  the  possessor 
too  conspicuous  for  his  safety,  though  this  appears  to  be  a 
consideration  of  no  consequence  in  the  case  of  skunks,  which 
possibly  are  less  disturbed  because  of  their  advertisement. 
But  under  artificial  selection  in  captivity  it  is  possible  rapidly 
to  increase  the  extent  of  the  white  areas  in  the  coat,which 
then  takes  on  striking  and  often  rather  definite  outlines,  as  in 
Dutch-marked  rabbits,  ''English"  rabbits  (Fig.  123),  hooded 
rats  (Fig.  56),  and  black-eyed  white  mice,  the  latter  being  all 
white  except  the  eyes.  The  production  of  white-spotted  races 
from  small  beginnings  observed  in  wild  stocks  has  been  ac- 
complished in  the  laboratory  by  Castle  and  Phillips  in  the 
case  of  Peromyscus  and  by  Little  in  the  case  of  the  house- 
mouse  (unpublished  data).  Physiologically  this  variation  is 
quite  distinct  from  the  albino  variation.  It  appears  to  be 
due  to  a  locally  inhibited  action  of  the  color  factor,  which  in 

* 

extent  that  the  prevaiHng  color  of  the  coat  is  black.  The  same  is  true  in  some  speci- 
mens of  the  black  rat  (Mus  rattus),  this  black  character  being  dominant  in  crosses 
over  the  true  agouti  character  found  in  the  gray  variety  of  the  same  species  which 
is  known  as  the  "roof-rat"  {Mus  Alexandrinus  of  some  systematists).  A  similar 
dominant  black  has  been  discovered  among  domestic  rabbits  by  Punnett,  who  has 
shown  that  it  owes  its  origin  to  a  change,  not  in  the  agouti  factor,  but  in  the  exten- 
sion factor,  E,  which  has  become  of  such  unusual  strength  or  potency  that  the 
agouti  factor  is  unable  in  its  presence  to  produce  the  usual  conspicuous  effect. 


126  GENETICS  AND  EUGENICS 

other  parts  of  the  body  retains  its  full  force;  whereas  in  an 
albino  the  action  of  the  color  factor  is  everywhere  wanting  or 
greatly  weakened. 

The  variation,  ** white  spotting,"  may  be  regarded  as  a 
unit-character  change  from  a  condition  of  uniform  action  of 
the  color  factor  to  a  condition  of  locally  suppressed  action  of 
the  color  factor.  The  former  may  be  designated  S,  the 
latter  s.  Its  inheritance  is  as  sharply  Mendelian  as  that  of 
any  other  color  variation  but,  the  precise  extent  to  which 
color  development  is  suppressed  being  obviously  quantita- 
tively variable  (Fig.  56),  it  is  easier  by  selection  to  modify 
the  modal  state  of  a  white-spotted  race  than  of  races  of  most 
other  color  varieties. 

That  this  factor  is  genetically  entirely  distinct  from  albi- 
nism is  shown  by  the  fact  that  w^hite-spotting  is  transmitted 
quite  as  readily  through  albinos  as  through  colored  indi- 
viduals. 

In  some  rodents  not  only  the  color  factor,  but  also  the 
extension  factor  is  subject  to  locally  inhibited  action.  Local 
inhibition  of  the  extension  factor  produces  yellow  spots  in  an 
otherwise  black,  brown,  or  agouti  coat.  This  color  variation, 
which  follows  Mendel's  law  in  crosses,  may  be  called  yellow 
spotting.  It  behaves  as  a  third  allelomorph  (eO  alternative 
both  to  full  extension  (E)  and  to  full  restriction  (e).  When 
yellow  spotting  coexists  with  white  spotting,  a  tri-color  con- 
dition of  the  coat  results,  spots  of  yellow,  white,  and  black 
(or  brown)  being  found  on  the  same  individual.  Familiar 
examples  are  found  among  guinea-pigs,  cats  and  dogs. 

Another  unit-character  variation  of  certain  rodents  greatly 
reduces  the  production  of  black  and  brown  pigments  without 
affecting  at  all  the  production  of  yellow  pigment.  As  the 
pigmentation  of  the  eye  consists  almost  entirely  of  black  or 
brown,  it  follows  that  in  this  variation  the  eyes  become  pink, 
while  the  coat  pigments  other  than  yellow  are  greatly  reduced 
in  amount.  Pink-eyed  blacks  or  browns  are  very  pale  coated, 
but  pink-eyed  yellows  are  indistinguishable  from  other  yel- 
lows except  by  the  eye-color.    The  changed  eye-color  is  ac- 


Fig.  57.  A  trihybrid  Mendelian  cross  between  a  black  hooded  rat  (top  left)  and  an  all-yellow  sport 
(top  right)  recently  captured  among  wild  Norway  rats  in  England.  Fi,  one  of  the  first-generation  pro- 
geny, gray  by  reversion,  like  wild  rats.  F2,  the  eight  classes  of  second-generation  young,  from  left  to 
right,  black  hooded,  black  self,  gray  hooded,  gray  self,  yellow  self,  yellow  hooded,  cream  (non-agouti 
yellow)  self,  cream  hooded.  Numerically  these  classes  should  beas3:9:9:27:9:3  :3:1.  Let 
the  reader  determine  which  of  the  eight  classes  may  be  expected  to  breed  true  and  to  what  extent  the 
other  varieties  will  not  breed  true  without  "  fixation  "  (elimination  of  heterozygotes) . 


UNIT-CHARACTERS  OF  RODENTS  127 

cordingly  the  most  constant  feature  produced  by  this  varia- 
tion. The  dominant  phase  of  this  unit-character,  which  is 
regularly  found  in  all  wild  races,  may  be  designated  dark-eye, 
D;  its  recessive  allelomorph,  pink-eye,  d.  The  recessive  varia- 
tion, pink-eye,  occurs  in  guinea-pigs,  rats,  and  mice.  It  has 
not  been  reported  as  yet  for  any  other  mammal.  (See  Fig. 
55.) 

Another  unit-character  variation,  which  affects  the  pig- 
mentation of  rodents,  occurs  also  in  other  mammals.  This 
consists  in  a  reduced  quantity  of  pigment  and  in  such  a 
clumping  of  the  pigment  granules  within  the  air  spaces  of  the 
hair  as  to  produce  a  dilution  of  the  pigmentation  as  a  whole. 
Black  under  these  circumstances  becomes  a  slaty  blue,  choco- 
late becomes  a  dull  muddy  brown,  and  yellow  acquires  a  pale 
washed-out  appearance.  The  best-known  examples  are  found 
in  blue  (Maltese)  cats,  blue  rabbits  and  blue  mice.^  This 
condition  may  be  regarded  as  a  recessive  variation  of  a  factor 
for  intense  pigmentation  normally  found  in  w^ild  rodents.  We 
may  designate  this  intensity  factor  by  I,  its  recessive  allelo- 
morph by  i  (dilution). 

In  guinea-pigs  and  rabbits  there  has  occurred  a  unit-char- 
acter variation  which  affects,  not  the  color,  but  the  length 
and  texture  of  the  hair,  which  in  the  so-called  *' angora" 
variety  is  long  and  silky.  This  results  from  a  failure  of  the 
hair  follicle  to  end  its  activity  when  the  hair  has  attained  its 
normal  length.  In  the  angora  variety  the  hair  keeps  on  grow- 
ing for  an  indefinitely  long  period.  The  long  or  angora  coat 
of  guinea-pigs  and*  rabbits  is  a  recessive  character  in  relation 
to  normal  (short)  coat.  We  may  regard  a  normal  and  domi- 
nant character  for  short  coat,  L,  as  having  undergone  varia- 
tion to  long  coat,  1.    (See  Figs.  36,  37,  and  41.) 

Among  guinea-pigs  alone  of  rodents  has  occurred  another 
morphological  unit-character  variation  of  the  coat,  which, 
instead  of  being  smooth  and  sloping  uniformly  from  the  nose 

^  This  variation  probably  does  not  occur  in  guinea-pigs;  what  was  at  one  time 
described  as  a  variation  of  this  sort  having  proved  to  be  an  alternative  form  of  the 
color  factor. 


128  GENETICS  AND  EUGENICS 

backward  as  in  wild  mammals,  may  become  rough  or  rosetted 
with  the  hair  radiating  out  from  centers  located  in  various 
parts  of  the  body.  (See  Fig.  33.)  Rough  coat  is  dominant 
over  smooth  coat,  for  w^hich  reason  we  may  consider  a  unit- 
character,  rough  coat,  R,  to  be  responsible  for  it,  the  recessive 
phase  of  which,  r,  is  found  in  smooth-coated  guinea-pigs. 

It  should  be  noted  that  both  rough  coat  and  short  coat, 
like  the  uniformity  factors  affecting  pigmentation,  obviously 
vary  quantitatively.  For  some  rough  guinea-pigs  are  rougher 
than  others  and  some  long-haired  guinea-pigs  have  longer, 
silkier  hair  than  others.  Selection  has  undoubtedly  been 
concerned  in  producing  the  present  high  standard  long-haired 
and  rough-coated  guinea-pigs  respectively.  Dr.  Sewall 
Wright  has  show^n  (Castle  and  Wright,  1916)  that  an  inde- 
pendent Mendelizing  factor  found  in  many  wild  cavies  inter- 
feres with  or  partially  inhibits  the  development  of  the  rough 
coat  in  hybrid  guinea-pigs.  We  may  designate  this  factor 
rough  modifier  (M),  its  recessive  phase  which  permits  full 
development  of  the  rough  coat  may  be  expressed  by  m.  Aside 
from  this  striking  modifier  of  rough,  it  is  probable  that  nu- 
merous other  factors  act  as  slight  modifiers  of  rough  and  that 
the  apparently  continuous  variation  in  the  development  of 
the  roughness  may  thus  be  accounted  for.  Continuous  varia- 
tion in  the  expression  of  the  angora  character,  as  regards 
length  of  hair,  may  be  accounted  for  on  similar  grounds. 

Leaving  out  of  consideration  such  quantitative  variations, 
it  is  possible  to  obtain  by  crosses  a  large  number  of  different 
unit-character  combinations  of  the  ten  independent  varia- 
tions which  have  been  mentioned  as  occurring  in  guinea-pigs. 
Theoretically  one  thousand  and  twenty -four  are  possible,  or 
if  we  count  separately  homozygous  and  heterozygous  com- 
binations, fifty-nine  thousand  and  forty-nine  are  possible. 
Needless  to  say  there  have  been  produced  thus  far  only  a  small 
part  of  the  varieties  of  guinea-pigs  theoretically  possible  as 
unit-character  combinations  of  the  ten  factorial  variations 
known  to  have  occurred  in  this  species.  And  the  variation 
of  the  guinea-pig  is  not  different  in  kind  or  degree  from 


UNIT-CHARACTERS  OF  RODENTS  129 

that  of  other  rodents.  Its  variation  has  probably  merely 
been  followed  up  more  closely  by  selective  breeding.  Among 
domesticated  rabbits,  at  least  seven  of  the  ten  enumerated 
variations  have  occurred;  all  except  the  pink-eye  and  the 
rough-coat  variations  are  reported  for  rabbits,  and  most  of 
them  are  well  known.  The  house  mouse  has  undergone  at 
least  six  of  the  ten  variations  listed  in  Table  12.  Its  yellow 
varieties  have  apparently  not  arisen  in  the  same  way  as  yel- 
low varieties  of  guinea-pigs  and  rabbits,  but  by  a  peculiar 
change  in  the  agouti  factor,  for  yellow  in  mice  is  a  third 
allelomorph  of  agouti  and  non-agouti.  Mice  also  lack  long- 
haired and  rough-coated  varieties,  but  in  other  respects  the 
variations  of  mice  are  parallel  with  those  of  guinea-pigs.  In 
the  Norway  rat  four  of  the  ten  unit-character  variations  of 
guinea-pigs  find  exact  equivalents,  viz.,  in  albinism,  non- 
agouti,  pink-eye  and  white  spotting.  A  third  allelomorph  of 
the  color  factor  (ruby-eye)  has  been  shown  by  WTiiting  and 
King  to  occur  among  wild  rats. 

A  red-eyed  yellow  variety  of  rats  is  due  to  a  unit-character 
variation  distinct  from  the  yellow  variations  known  in 
guinea-pigs  and  in  mice  respectively.  In  one  and  the  same 
linkage  system  in  the  Norway  rat  are  found  (1)  the  color 
factor  and  its  allelomorph,  ruby-eye,  (2)  the  factor  for  pink- 
eyed  yellow  and  (3)    the  factor  for  red-eyed  yellow. 


CHAPTER  XIII 

UNIT-CHARACTERS  IN  CATTLE  AND  HORSES 

Unit-character  changes  have  produced  new  varieties 
among  our  more  important  domesticated  mammals  as  well 
as  among  our  pet  rodents. 

Cattle.  Among  cattle  four  or  five  Mendelizing  color  varia- 
tions occur  similar  to  those  of  rodents  and  in  addition  two 
variations  of  a  morphological  character  have  been  reported, 
one  of  which  has  considerable  economic  importance.  Wild 
cattle  existed  within  historic  times  in  central  Europe,  the 
hunting  of  the  last-existing  herds  being  held  as  a  royal  pre- 
rogative by  the  kings  of  Poland.  These  cattle  represented 
probably  the  chief  source  from  which  domesticated  cattle 
were  derived.  They  were  of  large  size  but  of  what  color  we 
do  not  certainly  know.  It  seems  probable,  however,  that 
their  coat,  like  that  of  most  wild  ruminants,  contained  a 
mixture  of  yellow  and  black  pigments  somewhat  like  the 
coat  of  Jersey  cattle  at  the  present  time.  In  most  existing 
domestic  breeds  either  the  black  or  the  yellow  pigments  have 
become  predominant  or  white  has  taken  their  place  in  whole 
or  in  part.  Such  is  the  general  tendency  of  man's  agency  in 
modifying  the  color  characters  of  his  domesticated  animals. 
Nature's  colors  are  usually  adapted  to  concealment  or  pro- 
tection. Mixtures  of  pigments  are  common  and  minute  color 
patterns  abound.  Man  seeks  to  make  his  domestic  animals 
as  different  as  possible  from  the  wild.  He  either  gives  pre- 
ference to  pure  colors,  black,  white,  or  yellow,  or  seeks  to 
outdo  nature  in  the  production  of  color  patterns  in  great 
blotches  of  two  or  three  colors.  The  materials  for  his  opera- 
tions consist  of  sports  to  solid  black,  yellow,  or  white, 
together  with  white  spotting  and  yellow  spotting.  All  of 
these  have  occurred  among  cattle  and  have  been  used  to  the 
fullest  extent.  , 

130 


Fig.  58.     Wild  white  cattle  from  Chartley  Park,  England.     (After  Wallace.) 


Fig.  59.     Wild  white  cattle  from  Chartley  Park.     Note  black  individual  produced  by  white  parents. 

(After  Wallace.) 


Fig.  60.     Kerry  cow,  a  black  breed,  originated  in 

Ireland.     (Figs.  60  and  61  from  photographs  by 

Professor  C.  S.  Plumb.) 


Fig.    61.      Dexter-Kerry    cow.   Its  short-legged 

compact  form  is  a  dominant  Mendelian  character 

according  to  Professor  James  Wilson. 


Fig.  6^2.     White  short-horn  heifer. 


Fig.  63.     Red  short-horn  heifer  with  a  small  amount  of  white  spotting  underneath. 


Fig.  64.     Roan  short-horn  cow.    Beef  tjT)e.    The  fine  mosaic  of  red  and  white  spots  indicates  that  this 
animal  is  a  heterozygote  between  red  and  white  (Figs.  62  and  63). 


Fig.  65.  Ayrshire  bull.  Extensive  white  spotting  in  this  breed  leaves  only  an  occasional  small  s{X)t 
pigmented.  The  breed  is  hardy,  "  dual  purpose  "  but  inclining  more  to  the  dairy  type,  yet  less  spe- 
cialized and  better  adapted  to  a  severe  climate  than  the  Jersey  and  Guernsey  breeds.  It  originated  in 
Scotland, 


il  I' 


I 


r 


UNIT-CHARACTERS  OF  CATTLE  131 

In  English  parks  there  have  existed,  since  Roman  days  and 
perhaps  longer,  herds  of  all-white  cattle  kept  in  a  half  wild 
state.  Some  have  supposed  that  these  white  cattle  represent 
the  unchanged  original  stock  of  European  wild  cattle,  but  it 
seems  much  more  probable  that  they  represent  a  striking 
sport  from  the  original  stock,  which  was  isolated  and  allowed 
to  increase  in  the  hunting  preserves  of  princes,  a  semi-sacred 
character  perhaps  attaching  to  it.  These  cattle  differ  from 
albinos  among  rodents  in  that  they  have  pigmented  eyes. 

TABLE   13 

Some  Unit-Chara.cters  of  Cattle 

Dominant  Recessive 

Black.  Yellow.  ^      / 

Z^l  Polled.  Horned.  ,       f 

Dexter  form  (short  legs).  Kerry  form  (legs  normal). 

Dominance  Uncertain  or  variable 

White.  Colored. 

Uniformly  colored.  Spotted  with  white. 

Uniformly  black.  Black  spotted  with  yellow. 

They  also  have  some  sooty  black  or  brownish  pigment  in  the 
skin  and  hair  of  the  extremities  (feet,  nose,  ears,  and  tail). 
Ordinarily  they  breed  true,  but  occasionally  an  all  black  calf 
is  produced,  but  whether  as  a  recessive  in  the  Mendelian 
sense  or  as  a  reversion,  through  recombination  of  comple- 
mentary color  factors,  is  unknown.  (See  Figs.  58  and  59.) 
In  any  case  it  seems  highly  probable  that  the  white  race 
resulted  from  an  ancient  sport  derived  directly  from  wild 
cattle.  In  the  breed  of  "  short-horn  "  cattle,  which  origi- 
nated in  England,  white  individuals  frequently  occur  and 
they  breed  true  when  mated  with  each  other.  In  matings 
with  red  individuals,  a  sort  which  also  breeds  true,  roan 
heterozygotes  are  produced  (as  noted  on  page  110).  The 
white  of  this  breed  was  probably  derived  from  the  same 
original  source  as  the  white  cattle  of  the  English  parks,  but 
the  black  character  which  seems  to  inhere  in  the  cattle  of  the 
parks  has  been  eliminated  from  the  short-horn  breed,  which 
produces  only  reds,  whites,  and  their  heterozygotes,  with  or 
without  admixture  of  white  spotting.    (See  Figs.  62-64.) 


132  GENETICS  AND  EUGENICS 

Red  cattle  have  an  intensified  yellow  pigmentation.  They 
probably  represent  derivatives  of  an  original  all-yellow  sport, 
comparable  with  the  yellow  sports  of  rodents,  Which  originate 
through  restriction  of  black  pigment  to  the  eye.  Among 
cattle  yellows  vary  in  shade  from  a  very  deep  red  (Devons 
and  short-horns)  to  a  light  cream  color  (some  South  German 
and  Swiss  breeds).  The  extremes  in  both  directions  w^ere 
doubtless  secured  through  repeated  selection.  WTiether  the 
different  shades  or  intensities  of  yellow  are  alternative  is  un- 
known, but  it  seems  probable  that  in  cattle  as  in  rodents 
intensity  of  pigmentation  is  independent  of  its  specific  char- 
acter as  black  or  yellow. 

Black  breeds  of  cattle  are  represented  by  the  Galloway  and 
Aberdeen  Angvis  of  Scotland.  In  them  we  have  either  deriva- 
tives of  an  all  black  sport,  or  the  end  result  of  a  gradual  in- 
crease of  black  in  the  coat  through  selection.  Pure-bred 
Aberdeen  Angus  cattle  sometimes  produce  red  calves,  red 
being  obviously  a  Mendelian  allelomorph  recessive  to  black 
in  cattle  as  it  is  in  rodents.  As  red  is  not  favored  in  the 
standard  of  the  breed,  it  w^ill  doubtless  be  entirely  eliminated 
in  time,  as  seems  already  to  be  the  case  in  the  best  families 
of  the  Galloway  breed.     (See  Fig.  73.) 

In  most  breeds  of  cattle  white  spotting  occurs  and  this  is 
a  Mendelian  alternative  to  uniform  coloration,  though  nei- 
ther condition  is  entirely  dominant  over  the  other.  The 
self-coloration  of  breeds  which  are  all  black  or  all  white  has 
a  strong  tendency  to  prevail  in  the  offspring.  Black  breeds 
in  which  white  spotting  occurs  are  represented  by  the  Hol- 
stein-Friesian  cattle  originally  bred  in  Holland  and  Denmark, 
but  now  extensively  kept  in  this  country,  also  by  the  belted 
cattle  of  Holland.  (See  Figs.  66  and  69.)  Red-and- white 
and  yellow-and-white  cattle  are  represented  by  Hereford  and 
Guernsey  cattle  respectively.  (Figs.  68  and  67.)  Black- 
and-white  breeds  may  produce  red-and-white  offspring  as 
recessives,  but  red-and-white  breeds  never  produce  black- 
and-white  calves,  which  shows  clearly  that  black  is  dominant 
over  red.    In  the  Hereford  breed  a  definite  pattern  of  white 


Fig.  66.  Holstein-Friesian  cow  and  her  triplet  calves.  Note  the  black-and-white  mottling  similar  in 
all  four  animals,  yet  with  individual  differences.  This  breed  of  large  vigorous  cattle  originated  on  the 
borders  of  the  North  Sea  in  Europe.  It  excels  all  other  breeds  in  milk  production.  (Photograph  by  the 
owner,  N.  P.  Sorensen,  Bellingham,  Wash.) 


Fig.  67.  Guernsey  cow,  "  golden  yellow-and-white  "  in  color,  graceful  in  form,  gentle  in  dLsposition. 
producing  a  good  quantity  of  milk  extremely  rich  in  butter-fat.  The  breed  came  originally  from  the 
island  of  Guernsey.    (Photograph  from  Lang  water  farms,  N.  Easton,  Mass.,  F.  L.  Ames,  proprietor). 


i 


Fig.  68.  Hereford  heifer.  One  of  tlie  leading  beef  breeds,  dark  red  and  white  in  color.  The  white 
face,  back  stripe  and  underline  constitute  a  pattern  which  has  a  tendency  to  dominate  in  crosses.  (See 
Fig.  80a.)     Like  the  short-horn,  its  principal  rival  as  a  beef  breed,  this  breed  arose  in  England. 


Fig.  69.  Dutch  belted  or  "  Lakenfeld  "  bull.  Bred  for  three  centuries  for  this  chanut eristic  pattern 
by  aristocratic  families  of  Holland.  Probably  derived  from  the  same  original  general  stock  as  the  cattle 
of  Holstein  farther  east,  but  selected  more  closely  for  color  pattern  to  which  productiveness  has  been 
sacrificed. 


4 
^ 


m\ 


Fig.  70.     Polled  Durham  (or  short-horn)  cow.    Produced  by  a  hornless  sport  within  the  short-horn 
breed  or  possibly  by  some  unrecorded  cross,  as  with  the  polled  red  breed. 


Fig.  71.  Polled  Hereford  heifer.  A  breed  of  English  origin,  ik-liornod  in  Aiu.rira  i»y  tli.  .ii»i>lii-.itiuii 
of  genetic  principles.  Hornlessness  is  a  dominant  sport  or  "  mutation."  Compare  Fig.  68.  A  com- 
parison of  the  white  spotting  in  Figs.  70  and  71  suggests  strongly  that  one  is  only  a  more  advanced 
stage  (quantitatively)  of  the  other. 


UNIT-CHARACTERS  OF  CATTLE  133 

spotting  has  been  so  fixed  by  selection  that  it  shows  itself  (as 
a  white  forehead)  in  crosses  with  self-colored  breeds  and  even 
in  hybrids  with  the  American  bison. 

Yellow  spotting  on  a  black  background  is  not  very  com- 
mon among  cattle,  no  standard  breed  with  this  characteristic 
being  known,  but  a  brindling  of  yellow  and  black  spots  is  oc- 
casionally seen  in  mongrel  animals  and  no  doubt  good  black- 
and-yellow  spotted  animals  could  be  produced,  if  it  were 
considered  sufficiently  desirable,  or  even  tri-colors  with  black- 
yellow-and-white  coats.  It  is  possible  that  brindling  (yellow 
spotted  with  black)  is  a  third  allelomorph  of  black  and  of 
yellow,  as  in  guinea-pigs. 

A  morphological  variation  of  cattle  of  some  economic  im- 
portance is  hornlessness.  This  has  occurred  among  cattle  of 
Scotland  and  England  for  several  centuries  at  least  and  is 
known  also  to  have  occurred  among  cattle  kept  on  the  conti- 
nent and  still  earlier  to  have  occurred  among  cattle  of  the 
ancient  Egyptians.  Loss  of  horns  is  a  completely  discon- 
tinuous variation,  dominant  in  crosses.  Heterozygotes  may 
develop  mere  traces  of  horns,  known  as  scurs,  but  never  a 
fully  formed  horn  with  bony  core.  Hornlessness  has  become 
an  established  racial  character  (homozygous)  in  the  Scotch 
breeds  of  black  cattle,  Aberdeen  Angus  (Fig.  73)  and  Gallo- 
way, also  in  an  English  breed  of  red  cattle  called  Red 
Polled.  Within  the  last  thirty  years  polled  sports  have 
appeared  in  pure-bred  Holstein  cattle  in  the  United  States 
and  a  breed  of  polled  Holsteins  is  now  being  established  in 
this  country.  A  breed  of  polled  Herefords  was  produced  in 
the  United  States  from  a  three-quarters  Hereford,  one- 
quarter  short-horn  polled  calf  born  in  1889.  (Wallace,  p. 
122.)  See  Figs  68  and  71.  Polled  cattle  are  easier  to  manage 
and  less  liable  to  injure  each  other  than  are  horned  cattle. 
There  can  be  no  doubt  that  hornlessness  had  its  origin  as  a 
unit-character  variation  dominant  in  crosses. 

Another  morphological  character,  said  to  be  a  Mendelian 
dominant,  occurs  in  Dexter-Kerry  cattle.  They  have  ab- 
normally short,  stumpy  legs.    (See  Figs.  60  and  61.) 


134  GENETICS  AND  EUGENICS 

Horses.  The  original  color  of  wild  horses  is  probably  seen 
in  a  wild  horse  still  existing  on  the  plains  of  central  Asia 
(Mongolia)  and  known  as  Prevalski's  ^  horse.  (See  Fig.  81.) 
It  has  somewhat  the  appearance  of  an  ordinary  bay  horse, 
except  that  the  yellow  pigment  is  paler  and  the  black  pig- 
ment more  diffuse  dorsally.  The  mane,  tail  and  legs  are  black, 
the  back  reddish  or  yellowish  brown  shading  off  into  pale 
sooty  yellow  below.  In  tame  horses  of  the  bay  color  variety 
as  compared  with  this,   the  yellow  pigmentation  is  of  a 

TABLE   14 


I 


Some  Unit-Characters  of  Horses 

Dominant  Recessive 

1.  Bay.  Not  bay  (i.  e.,  black  or  chestnut). 

2.  Black.  Chestnut. 

3.  Gray.  Not  gray  (any  color  but  gray). 

4.  Trotting.  Pacing. 

Dominance  Uncertain  or  Wanting 
o.   Uniformly  colored.  Spotted  with  white. 

brighter  and  more  intense  sort,  called  red,  and  more  free 
from  black  dorsally,  while  the  black  markings  of  mane,  tail, 
and  feet  are  probably  more  distinct,  changes  that  seem  to 
have  come  in  with  careful  selective  breeding.  For  in  mongrel 
horses  of  no  particular  breeding  the  fine  points  of  the  bay  are 
often  wanting,  the  yellow  being  of  a  dull  shade  and  mixed 
dorsally  with  black  and  approaching  a  "  dun  "  in  general 
appearance.  Unit-character  variations  are  less  in  evidence 
in  domestic  horses  than  in  cattle.  The  bay  appears  to  be  an 
improved  ty^e  of  wild-horse  coloration  not  produced  by 
abrupt  changes  in  any  particular  characters  but  by  gradual 
changes  in  several  characters.  Black  is  a  color  variety  reces- 
sive to  bay  in  crosses.  It  seems  to  have  arisen  in  the  same 
way  that  black  varieties  of  rodents  usually  arise,  by  loss  of  a 
pattern  factor.  In  rodents  it  is  the  agouti  factor  which 
having  disappeared  produces  a  black  (non-agouti)  variety. 

^  The  common  spelling  of  this  name  is  Prejvalski,  but  as  this  makes  in  English 
an  unpronounceable  combination,  I  take  the  liberty  of  dropping  the  j  in  the  interest 
of  my  readers,  without  intentional  disrespect  to  Mr.  Prejvalski  or  his  horse. 


Fig.  72.  Jersey  cow.  One  of  the  best  strictly  dairy  breeds.  Color  light  yellow  ("  fawn  ")  shaded  with 
diffuse  black  pigment,  possibly  a  primitive  type  of  coloration  in  cattle.  Similar  to  the  Guernsey  in 
character  and  source.  Home  the  island  of  Jersey.  A  little  delicate  in  constitution  and  nervous  in 
temperament. 


Fig.  73.     Polled  Aberdeen  Angus  bull.    A  Scotch  breed,  self  black  in  color,  of  beef  type  aiul  hanly. 


I 


Fig.  74.     Fi  cow,  black,  polled. 


Fig.  76.     Selected  F2  bull. 


Fig.  78.     Rejected  F2  buU. 


Fig.  75.     Choice  F2  heifer. 


Fig.  77.     Selected  F2  cow. 


ijected  Fi  heifer. 


Results  of  crossing  Jersey  cows  (Fig.  72)  with  an  Angus  bull  (Fig.  73)  in  an  effort  to  combine  in  one  race 
the  dairy  excellence  of  the  former  with  the  size,  hardiness  and  good  feeding  qualities  of  the  latter. 
Figs.  74,  78  and  79  show  the  dominant  black  of  the  Angus,  Figs.  75-77  show  the  recessive  fawn  of  the 
Jersey  somewhat  darkened.    All  show  dominant  hornlessness.    (After  Kuhlman.) 


i 


Fig.  80.     A  zebu  bull,  typical  example  of  one  of  the  humped  cattle  of  India.    (Photograph  from  Pro- 
fessor Nabours,  Kansas  Agr.  College.) 


riG.  80a.  Fi  calf  from  cross  of  zebu  bull  with  Hereford  cow.  Notice  imperfect  dominance  of  Hereford 
pattern  (Fig.  68).  Indian  cattle  being  more  tolerant  of  heat  and  more  resistant  to  Texas  fever,  the  cross 
IS  made  to  combine  these  qualities  with  the  beef  excellence  of  the  Hereford.  (Photograph  from  Na- 
bours.) 


fl 


UNIT-CHARACTERS  OF  HORSES  135 

In  horses  it  is  a  hay  factor  which  the  black  variety  has  lost. 
This  factor  appears  to  inhibit  the  development  of  black  in 
regions  where  the  bay  variety  shows  red,  just  as  an  agouti 
factor  inhibits  the  development  of  black  pigment  in  certain 
regions  of  the  coat  of  rodents  which  then  are  yellow.  Wlien 
the  bay  factor  is  lacking,  black  pigment  develops  throughout 
the  entire  coat.  Whether  this  loss  occurred  originally  as  a 
single  sudden  change  (a  sport)  or  whether  it  occurred  gradually 
is  uncertain,  but  it  seems  clear  that  at  present  in  crosses  black 
is  a  unit-character  recessive  to  bay,  and  this  makes  it  seem 
probable  that  it  arose  as  a  discontinuous  variation  originally. 

A  unit-character  difference  has  also  been  shown  to  exist 
between  black  and  chestnut  horses,  a  difference  comparable 
to  that  which  exists  between  black  and  brown  varieties  of 
rodents.  Chestnut  is  recessive  to  black,  corresponding  with 
the  "  chocolate  "  varieties  of  rodents.  "  Suffolk  "  or  "  Suf- 
folk Punch  "  horses  are  invariably  chestnut  in  color.  But 
the  term  "  chestnut  "  as  here  used  probably  includes  both 
brown  animals  which,  like  black,  lack  the  bay  factor  and  those 
which  possess  this  factor.  For  the  latter  it  would  probably 
be  better  to  use  a  term  in  common  use,  sorrel.  We  should 
then  have  parallel  black  and  brown  series  with  and  without 
the  bay  factor.  Black  pigmented  horses  with  the  bay  factor 
are  "  bays,"  without  it  they  are  "  blacks."  BrowTi  pig- 
mented horses  with  the  bay  factor  should  be  called  "  sor- 
rel ";  those  without  it,  chestnut.  Records  compiled  by 
Wentworth  and  others  indicate  that  such  a  factorial  differ- 
ence does  exist  among  horses  called  "  chestnut  "  in  the 
records.  For  blacks  mated  inter  se  produce  some  chestnut 
colts  (which  should  be  possible  if  the  black  parents  are 
heterozygous  for  chestnut)  with  a  doubtful  record  of  a  few 
bays,  but  black  mated  with  "  chestnut  "  produces  more  bays 
than  anything  else,  which  shows  clearly  that  some  at  least 
of  the  chestnut  parents  do  transmit  the  bay  factor. 

The  gray  (or  white)  color  variation  of  horses  corresponds 
roughly  with  the  white  variation  in  cattle.  It  is  a  dominant 
unit-character  in  crosses,  but  shows  itself  only  in  the  second 


136  GENETICS  AND  EUGENICS 

and  later  coats.  For  the  colts  are  born  with  colored  coats, 
but  at  the  iirst  shedding  of  the  hair,  white  hairs  begin  to 
come  in  mingled  with  the  colored  ones.  (See  Fig.  84.)  Later 
white  hair  may  almost  completely  replace  the  colored  ones. 
The  eyes  of  gray  horses  are  always  colored.  The  term  gray 
as  applied  to  horses  has  the  same  significance  as  when  applied 
to  human  beings.  It  means  the  occurrence  of  white  hairs 
among  colored  ones,  more  or  less  completely  replacing  them. 
WTien  among  horses  the  original  coat  partially  replaced  by 
white  was  a  black  one,  an  ordinary  or  ''  iron  "  gray  coat 
results;  but  when  the  original  coat  was  bay  or  sorrel,  then  a 
roan  coat  is  produced. 

White  spotting  is  of  frequent  occurrence  among  horses, 
though  it  is  usually  less  extensive  than  among  cattle.  In  this 
variation  the  loss  of  pigment  from  the  body  area  affected  is 
complete  and  is  present  from  birth  on,  so  that  its  nature  is 
evidently  very  different  from  the  gray  variation  already 
described.  (Figs.  81-85.)  It  corresponds  physiologically 
with  white  spotting  in  cattle  and  in  rodents.  The  com- 
monest form  of  white  spotting  is  the  occurrence  of  a  white 
spot  in  the  forehead  sometimes  extending  down  over  the  nose, 
or  the  possession  of  one  or  more  white  feet,  or  both.  These 
are  regular  features  of  the  coloration  of  Clydesdale  and  Shire 
horses.  More  extensive  spotting  takes  the  form  of  irregular 
white  areas  extending  across  the  neck  or  body.  (Fig.  81a.) 
It  is  less  common  than  the  former  and  unlike  it  behaves  as  a 
dominant  character  in  crosses.  Often  seen  in  children's 
ponies,  it  is  probably  genetically  distinct  from  the  spotting 
of  horses  with  white  stockings  and  blaze.  The  pacing  gait  in 
American  race  horses  is  a  character  recessive  to  the  trotting 
gait,  according  to  Bateson.  In  pacing  the  two  legs  of  the 
same  side  of  the  body  move  in  unison  or  nearly  so,  while  in 
trotting  the  foreleg  of  one  side  moves  almost  simultaneously 
with  the  hind  leg  of  the  other  side.  Some  trotters  may  be 
made  to  acquire  the  pacing  gait  and  these,  of  course,  may 
produce  trotters,  but  natural  pacers  produce  only  natural 
pacing  colts  when  bred  with  each  other,  whereas  in  crosses 
trotting  dominates. 


Fig.  81.  Prevalski  horse  in  the  New  York  Zoological  Garden.  (Photograph  by  courtesy  of  Director 
W.  T.  Hornaday.)  Notice  large  head,  erect  mane,  absence  of  forelock  and  taillock,  faint  zebra-like 
striping  on  front  leg,  and  general  pattern  of  "  bay,"  with  light  muzzle  and  darker  mane,  tail,  and  legs. 


^y" 


Fig.  81a.  Pony  of  uncertain  pedigree  on  farm  of  Simpson  Bros.,  Palmer,  III.  (Photograph  by  courtesy 
of  Professor  J.  A.  Detlefsen.)  Notice  general  form  like  that  of  Prevalski  horse,  but  with  white  spotting 
extending  up  over  front  legs  and  entirely  around  body.  Spotting  of  hind  feet  also  extends  up  over  l>ody 
on  right  side. 


Fig.  82.  A  saddle  horse  ("  hunter  ")  showing  typical  white  markings,  "  white  stockings  "  and 
"  blaze  "  (face  stripe).  These  are  manifestations  of  white  spotting  fully  developed  at  birth  and  not 
changed  subsequently. 


Fig.  83.     Clydesdale,  typical  example  of  one  of  the  breeds  of  heavy  draft  horses. 

blaze  of  white  are  regularly  present  in  this  breed. 


White  stockings  and 


[ 


Fig.  84.  Gray  Percheron  mare  anci  c-oit.  ^^uc•h  coits,  blacK  at  Dinh,  oecome  gray  later  iii  iite.  Notice, 
however,  that  the  colt's  face  is  already  white.  This  is  due  to  white  spotting,  as  in  the  hunter  and 
Clydesdale,  not  to  the  gray  factor.    The  two  forms  of  white  are  genetically  quite  distinct. 


Fig.  85.  White  mare  and  colt.  (Photograph  by  courtesy  of  W.  P.  Newell,  Washburn, 
111.)  An  extreme  condition  of  white  spotting  is  here  shown,  in  which  the  entire  coat  is 
white  from  birth  on. 


I 


CHAPTER  XIV 

UNIT-CHARACTERS  IN  SWINE,  SHEEP,  DOGS,  AND  CATS 

Swine.  In  the  wild  boar  of  Europe,  from  whicli  in  part 
domestic  swine  are  descended,  the  coat  is  slaty  black,  the 
individual  bristles  bearing  a  band  of  pale  yellow  like  the 
agouti  marking  of  rodents.  The  young  of  the  wild  boar  are 
also  marked  with  longitudinal  body  stripes,  a  character  per- 
haps correlated  with  the  agouti-like  banding  of  the  bristles. 
This  banded  character  of  both  young  and  adult  has  appar- 
ently been  lost  in  all  domestic  breeds,  which  are  either  self 
black,  red,  or  white,  or  else  black  or  red  spotted  with  white, 
yet  it  occasionally  reappears  in  crosses,  showing  a  probable 
dependence  upon  complementary  factors  still  found  sepa- 
rately in  certain  breeds  (Severson) .  In  the  white  variety  the 
entire  coat  is  colorless  but  the  eye  is  colored.  This  is  a  domi- 
nant variation.  White  spotting  is  possibly  a  distinct  varia- 
tion from  the  foregoing,  and  uncertain  as  to  dominance.  But 
it  may  be  that  the  two  differ  only  in  degree  and  are  really 
allelomorphs. 

TABLE  15 

Unit-Characters  of  Swine 
Dominant  Recessive 

1.  Wild  color.  Not  wild  color  (black  or  red). 

2.  Black.  Red. 

3.  Self  white.  Colored. 

4.  Mule-footed  (syndactyl).  Normal  foot. 

Dominance  Uncertain  or  Wanting 

5.  Uniformly  colored.  Spotted  with  white. 

Two  forms  of  white  spotting  (which  occur  naturally  and  are 
comparable  with  the  two  types  of  white  spotting  amouii 
horses)  are  sought  after  by  breeders  and  have  become  breed 
characters,  viz.,  (1)  a  condition  in  which  a  broad  wliite  belt 
encircles  the  body  (as  in  Hampshire  hogs)  and  (2)  a  condition 
in  which  white  appears  at  the  extremities,  on  the  feet  and 

137 


138  GENETICS  AND  EUGENICS 

snout  (as  in  Berkshires).  It  is  probable  that  they  are  similar 
in  genetic  character.  Black  among  swine  is  dominant  over 
red,  as  in  cattle,  horses  and  rodents.  But  the  dominance  of 
black  is  commonly  imperfect  or  complicated  by  the  presence 
of  a  spotting  factor  in  the  red  breeds  known  as  Tamworth 
and  Duroc- Jersey.     (See  Figs.  86-93.) 

A  curious  morphological  variation,  syndactylism,  is  a  domi- 
nant unit-character.  In  this  variation  the  normal  two  hoofs 
of  each  foot  have  completely  fused  together  and  the  foot  has 
a  single  hoof  like  a  ''mule."  Hence  the  variety  is  called 
'* mule-footed."  A  breed  having  this  characteristic  has  been 
established  in  the  United  States.  Although  the  hoofs  are 
fused  the  bones  proximal  to  the  toe  retain  their  original 
paired  character.    (See  Figs.  94  and  95.) 

Sheep.  In  sheep  ordinary  white  fleece  is  dominant  over 
black  fleece,  the  latter  occasionally  cropping  out  in  flocks  as 
a  recessive,  as  indicated  in  the  old  saying  *' every  flock  has 
its  black  sheep."  Black  sheep  breed  true  inter  se.  Black  is 
probably  not  a  reversionary  variation  but  a  loss  variation  of 
a  pattern  factor  found  in  wild  sheep  and  similar  to  the  bay 
pattern  of  horses.  Wild  sheep  are  white  or  whitish  except  at 
the  extremities  where  the  pigmentation  is  heavier.  In  some 
breeds  of  sheep  the  skin  and  wool  of  the  extremities  is  dark, 
similar  to  the  coat  of  Himalayan  rabbits,  and  white  spotting 
may  affect  these  pigmented  regions  just  as  it  does  the  coat 
of  Himalayan  rabbits.  (See  Figs.  96-100.)  Hornlessness  is 
a  variation  from  the  original  horned  condition  of  wild  sheep 
which  is  dominant  in  females  but  recessive  in  males,  a  matter 
deserving  further  consideration  in  connection  with  the  sub- 
ject of  heredity  as  affected  by  sex.    (See  Figs.  96-104.) 

Dogs.  By  Darwin  and  most  other  students  of  the  origin 
of  dogs,  the  conclusion  has  been  reached  that  dogs  are  de- 
scended from  several  different  wild  species  of  wolves  inde- 
pendently domesticated  in  different  parts  of  the  world. 
These,  it  was  thought,  having  been  subsequently  inter- 
crossed have  produced  a  highly  variable  stock  from  which 
selection  has  isolated  the  genetically  diverse  modern  breeds. 


Fig.  86.     Berkshire  boar.     Black  with  white  points. 


Fig.  87.     Yorkshire  boar.     A  self  white  breed. 


Fig.  88.     Fi  sow  from  cross,  Berkshire  X  Yorkshire,  and  F2  pigs.     Note  reappearance  of  recessive 
blacks  but  with  white  spotting  increased  in  amount.     (After  W.  W.  Smith.) 


Fig.  89.     Fi  sow  from  cross,  Berkshire  X  Yorkshire,  and  pi^^s  protiuceil  Ijy  a  l>ack-eross  with 
hour.     Note  1  :  1  ratio  and  modified  ^pol^tiiiL'.     lAftcr  W.  W.  ^tuilh.) 


Berkshire 


Fig.  90.     Hampshire  sow,  typical  example  of  a  belted  black-and-white  breed. 


Fig.  91.     Belted  red  sow.    This  breed  produced  by  Q.  I.  Simpson  by  crossing  black  belted  (Hampshire) 

with  self  red  (Tamworth  and  Duroc)  swine. 


I 


Fig.  92.     A  litter  of  pigs  by  two  belted  red  parents.    Evidently  this  form  of  white  spotting  is  not  fully 
recessive,  since  part  of  the  pigs  are  not  belted.     (By  courtesy  of  Simpson  and  Detlefsen.) 


Fig.  93.   A  belted  red  sow  and  her  litter  by  a  belted  red  boar.    Note  variation  in  belt  or  its  total  ab- 
sence.    (By  courtesy  of  Simpson  and  Detlefsen.) 


Figs.  94  and  95.    Foot  bones  of  mule-footed  (syndactyl)  swine.    Only  the  hoof  and  nearest  pair  of  bones 

show  complete  fusion.     (After  Spillmau.) 


UNIT-CHARACTERS  OF  DOGS  139 

A  different  opinion  as  to  the  ancestry  of  dogs  has  recently 
been  expressed  by  G.  S.  Miller  and  particularly  by  G.  M. 
Allen,  who  has  made  a  careful  study  of  the  cranial  characters 
of  dogs  kept  by  the  aboriginees  of  the  American  continent. 
Allen  finds  strong  evidence  that  the  native  dogs  of  America 
are  not  descended  from  American  wolves  but  came  with  man 
in  his  migration  from  north-eastern  Asia  to  north-western 
America.  Previous  to  that  migration  there  existed  in  Europe 
and  Asia  both  a  large  and  a  small  type  of  dog,  and  both  types 
were  introduced  into  America  when  it  was  peopled  from 
Asia.  A  third  type,  the  Eskimo  dog  with  heavy  coat  and 
tail  curled  forward  over  the  hip,  occurs  in  the  northernmost 
parts  of  both  Asia  and  America  and  doubtless  came  with  the 
Eskimos  in  their  comparatively  recent  migration  from  Asia. 
What  part,  if  any,  species  hybridization  has  played  in  the 
genesis  of  dogs  can  not  at  present  be  stated,  but  a  survey  of 
existing  breeds  of  dogs  shows  the  occurrence  among  them  of 
several  unit-characters  and  accordingly  unit-character  varia- 
tion (mutation)  may  be  regarded  as  having  been  an  import- 
ant element  in  their  production.  A  case  which  well  illus- 
trates the  point  is  the  color  variation  of  Great  Danes  as 
worked  out  by  Little  and  Jones.  (See  Fig.  104a.)  Starting 
with  the  self  black  variety  (3, Fig.  104a),  we  have  as  its  reces- 
sive allelomorphs  either  brindle  (4)  or  fawn  (5) .  A  recessive 
dilution  factor,  if  present  in  a  homozygous  condition,  gives 
us  dilute  black  (6),  dilute  brindle  (7)  and  dilute  fawn  (8). 
A  dominant  factor  for  white  spotting  produces  the  harlequin 
variety  (2).  A  recessive  factor  for  white  spotting  produces 
white  feet  or  breast  spot  (1).  These  t^^^es  of  white  spotting 
remind  us  respectively  of  the  English  and  Dutch  patterns  of 
white  spotting  among  rabbits.  Presumably  either  pattern 
might  occur  in  association  with  dilute  black,  brindled,  dilute 
brindled,  fawn,  or  dilute  fawn  coat  (4-8,  Fig.  104a).  AVliat 
are  probably  more  developed  forms  of  the  recessive  type  of 
white  spotting  are  represented  in  Figs.  100-109.  In  the 
breeds  there  shown  white  spotting  has  been  selected  lor, 
whereas  in  the  Great  Dane  it  is  rigidly  selected  against.    A 


140  GENETICS  AND  EUGENICS 

more  specialized  form  of  the  dominant  spotting  (harlequin) 
is  found  in  the  coach-dog  (Fig.  110).  Besides  the  five  unit- 
character  variations  of  Great  Danes,  several  other  unit- 
character  variations  can  be  recognized  in  other  breeds.  (See 
Table  16.)  The  cranial  characters  of  dogs  show  their  an- 
cestors to  have  been  wolf -like. 

Most  wolves  have  a  protectively  colored  gray  coat,  in 
which  black  and  yellow  pigments  are  intermingled  on  the 
same  hair  somewhat  as  in  the  agouti  pattern  of  rodents. 
This  pattern  is  wanting  in  most  dogs,  but  has  been  retained 
in  some  examples  of  the  Eskimo-dog  or  "husky."  It  is 
probably  due  to  a  dominant  factor. 

A  more  conspicuous  pattern  is  seen  in  black-and-tan  dogs. 
In  a  black-and-tan  the  general  body-color  is  yellow  (tan)  but 
with  a  blanket  of  black  extending  down  from  the  back  over 
the  sides  of  the  body  and  the  outer  surfaces  of  the  legs.  A 
yellow  spot  is  found  also  above  each  eye.  Fox  hounds  and 
beagles  have  this  pattern  regularly.  Airedale  terriers  are 
distinguished  chiefly  by  this  pattern  from  Irish  terriers. 
Some  setters  and  pointers  have  it  while  others  do  not.  Al- 
though the  white  spotting  in  these  breeds  often  obscures  it, 
the  black-and-tan  pattern  can  readily  be  recognized  in  the 
light  spot  above  the  eye.  It  is  apparently  a  recessive  pattern 
factor  in  various  breeds  of  dogs.  Since  the  pattern  seen  in 
black-and-tan  dogs  may  be  transferred  in  crosses  as  a  unit- 
character  to  dogs  which  are  brown  or  red  pigmented,  it  is 
probably  better  to  adopt  for  it  a  term  appropriate  in  different 
combinations.  Bi-color  has  been  suggested  by  Barrows  and 
Phillips  as  such  a  term.  Bi-color  black  dogs  are  "black-and- 
tan,"  bi-color  brown  dogs  are  "liver-and-tan,"  and  bi-color 
red  dogs  are  "red-and-lemon."  Self  black  breeds  of  dogs 
have  probably  originated  by  a  loss  of  an  original  pattern 
factor  such  as  the  bi-color  factor;  and  self  yellow  (  or  red) 
breeds  by  independent  loss  (sudden  or  gradual)  of  black  from 
the  coat.  Brown  ("liver")  varieties  have  originated  by  a 
unit-character  variation  from  black  to  brown,  comparable 
with  that  of  various  rodents.     Self  white  occurs  in  dogs 


Fig.  96.     "  Black  faced  "  Highland  ram.     (After  Plumb.) 


Fig.  97.     Black  faced  Highland  ram  and  ewes.    Note  white  spotting  of  pigmented  face  and  legs, 
also  sexual  difference  in  size  of  horns.     (After  Plumb.) 


Fig.  98.     Malitch  sheep.     An  Asiatic  flock  containing  self-black,  spotted  black-and-white  and  grayish 
white  sheep,  the  last  probably  the  primitive  condition,     (.\fter  C.  C.  Young.) 


Fig.  99.     Cheviot  ram.    This  Scotch  breed  has  long  and  coarse 
wool  with  face  and  legs  bare  and  white.    Both  sexes  are  hornless. 


Fig.  100.     Hampshire  Down  ewe.     Extremities  pigmented. 
Hornless  in  both  sexes. 


Fig.  101.     Delaine  merino  ram.     This  breed  produces  abundant,  fine  wool.      Males  have 
well-developed  horns,  females  are  hornless.     (Figs.  99-101  after  Plumb.) 


UNIT  CHARACTERS  OF  DOGS  141 

either  as  a  sport  from  the  colored  condition,  or  more  probably 
as  an  extreme  form  of  white  spotting.  In  this  variety  the  eye 
pigmentation  is  never  entirely  lost  as  in  albino  rodents;  it  is 
largely  retained,  as  is  the  case  also  in  white  cattle,  horses  and 
swine.  In  crosses  between  the  different  colored  breeds, 
black-and-tan  {i.  e.,  bi-color  black)  is  dominated  by  self  black 
and  bi-color  brown  by  self  brown;  black  is  dominant  over 
yellow  (or  red)  and  also  over  brown.  As  yellow  and  brown 
are  independent  unit-character  variations  they  may  be  com- 
bined, a  result  seen  in  brown-eyed  yellow  dogs.  Thus  among 
pointers  (Little,  1914)  or  cocker  spaniels  (Barrows  and 
Phillips,  1915)  a  cross  of  black-eyed  yellow  with  brown  pro- 
duces in  Fi  black  dogs  and  in  F2  blacks,  browns,  black-eyed 
yellows  and  brown-eyed  yellowsX  The  same  result  in  both 
Fi  and  F2  may  be  obtained  by  crossing  black  with  brown- 
eyed  yellow.  What  appears  to  be  self  white,  but  is  more 
probably  a  very  pale  yellow,  according  to  Barrows  and 
Phillips,  has  appeared  in  spaniels  as  a  sport  and  is  recessive 
in  heredity.  Whether  in  other  breeds  self  white  is  recessive 
or  dominant  is  not  known  at  present.  It  is  probable  that  in 
some  cases,  as  in  bull  terriers,  it  is  only  an  extreme  form  of 
white  spotting,  in  which  case  we  should  expect  the  dominance 
to  be  imperfect.  A  short  stumpy  tail  is  probably  a  dominant 
unit-character  variation  in  dogs,  as  it  is  in  cats. 


/ 


TABLE  16 

Unit-Characters  of  Dogs 

1. 

Gray. 

Black. 

2. 

Self-color. 

Bi-color  (black-and-tan, 
brown-and-tan,  red- 
and-tan). 

3. 

Black. 

Brindle,  yellow,  or  red  ^ 

4. 

Black. 

Brown  (liver). 

5. 

Harlequin  type  of  white 

Self  color. 

spotting. 

6. 

Color  intense. 

Color  dilute- 

7. 

Colored  all  over 

Spotted  with  white  (Dutch  type) 

8. 

Stumpy  tail. 

Normal  tail. 

^  In  Dachshunds  red  is  not  uniformly  recessive;  it  apparently  may  be  dominant. 


142  GENETICS  AND  EUGENICS 

Cats,  Domestic  cats  are  descended  from  a  wild  species 
{Felis  maniculata)  still  found  in  northern  Africa.  The  domes- 
tication was  accomplished  in  ancient  Egypt  and  the  domestic 
cat  was  introduced  into  Europe  in  the  middle  ages,  since 
Roman  times.  The  wild  species  is  similar  in  size  and  color  to 
the  common  tabby  or  tiger  cat.  This  has  a  coat  consisting 
of  agouti-like  hairs,  which  contain  both  black  and  yellow 
pigments,  but  the  body  is  marked  with  stripes  in  which  black 
pigment  predominates,  and  it  is  these  black  stripes  that  pro- 
duce the  tiger  pattern,  which  is  a  dominant  unit-character. 
Different  forms  of  the  tiger  pattern,  distinguished  as  lined, 
striped,  blotched,  etc.,  are  probably  multiple  allelomorphs. 
In  the  self -black  variety  the  tiger  pattern  and  agouti  mark- 
ing of  the  hairs  have  been  covered  up  by  a  greatly  increased 
amount  of  black.  The  black  variety  probably  originated  as 
a  sport  and  it  behaves  as  a  recessive  to  tabby.  An  all  yellow 
variety  represents  another  unit-character  variation  imper- 
fectly dominant  over  black.  Homozygous  individuals  are 
all  yellow  but  heterozygous  females  usually  show  both  yel- 
low and  black  (tortoise  shell)  though  occasionally  they  may 
be  all  yellow.  The  inheritance  of  yellow  is  sex-linked  and 
of  the  Drosophila  type.  (See  Chapter  XVIIl.)  Yellow  cats 
usually,  if  not  always,  show  the  tiger  pattern,  which  leads 
to  the  question  whether  this  pattern  is  ever  lost  even  in  the 
black  variety.  It  may  be  only  covered  up  with  black  pig- 
ment. Darwin  notes  the  fact  that  black  kittens  often  show 
the  tiger  pattern  which  is  not  visible  in  them  later  in  life. 
All-white  varieties  of  cats  exist  having  colored  eyes  (either 
"yellow"  or  blue).  The  relation  of  this  variation  to  colored 
forms,  as  regards  dominance,  is  uncertain,  but  it  probably 
represents  an  extreme  form  of  white  spotting.  Blue  (or 
Maltese)  is  a  dilute  form  of  black,  recessive  to  the  latter. 
The  dilution  factor  probably  affects  the  appearance  of  tabby 
and  yellow  also,  but  definite  information  on  the  point  is  not 
available.  White  spotting  is  a  character  the  behavior  of 
which  as  regards  dominance  is  unknown.  Yellow  spotting 
occurs  only  as  a  heterozygous  character  in  the  cross  between 


Fig.  105.     Pomeranian,  self-colored,  and  having 
long  silky  hair.     Toy  variety. 


Fig.  106.     Boston  hull  terrier.    Pattern  in  wiiili- 
spotting  like  the  Dutch  marking  of  rahbits. 


Fig.  107.     Saint  Bernard. 


Fig.  108.   Beagle.   Tri-color,  black-and-tan 
with  white. 


Fig.  109.     Collie.     Figs.  106-109  show  white  spotting  of  the  same  general  character. 


Fig.  110.     Dalmatian  or  coach  dog.    A  peculiar  form  of  white  spotting,  resembling  that  of 

the  English  rabbit,  is  found  in  this  breed. 


."■■A 


ii>- 


^,.it^.:';^,  u> 


Fig.  111.     Great  Dane.     Brindled  type,  with 
yellow  spotting  on  a  black  background. 


Fig.  ll^.     Irish  setter.     Color,  dark  red. 


Fig.  113.     Dachshund.     Black-and-tan. 

(Figs.  105-114,  by  courtesy  of  F.  G.  Carnochan,  from  Field  and  Fancy.) 


Fig.  114.     Bull  terrier.     All  white  except  nose 
and  eyes. 


vei 


In 


UNIT-CHARACTERS  OF  CATS  143 

yellow  and  black  and  then  chiefly  in  the  female  sex.  Lon^ 
(angora)  hair  is  a  recessive  variation  from  normal  coat  in  cats 
as  in  rabbits  and  guinea-pigs.  A  short  stumpy  tail,  seen  in 
the  ''Manx"  cat,  represents  an  imperfectly  dominant  unit- 
character  variation.  Homozygous  dominants  are  tailless; 
heterozygotes  are  short-tailed;  normal  (long)  tail  is  recessive. 
Polydactylism  (the  possession  of  extra  toes)  is  an  imperfectly 
dominant  variation. 


TABLE  17 

Unit-Characters  of  Cats 

Dominant                                                               Recessive 

1. 

Tabby.                                               Not  tabby  (black  or  blue). 

2. 

Black.                                               Blue. 

2a. 

Normal  (intense)  pigmentation.     Siamese  dilution. 

3. 

Short  hair.                                         Long  hair  (angora). 

Dominance  Imperfect  or  Uncertain 

4. 

Colored  all  over.                                Spotted  with  white. 

o. 

White  (eyes  only  colored).             Colored  all  over. 

6. 

Yellow.                                               Not  yellow  (tabby  or  black) 

7. 

Tailless  (Manx).                               Long-tailed. 

8. 

Polydactyl.                                      Toes  liormal. 

i-i'ivste  Propsrty  of 

Z.  p.  IViETCALF: 
No,. 


•'■iifimfSimfdt 


CHAPTER  XV 

UNIT-CHARACTERS  IN  POULTRY  AND  IX  PLANTS 

Poultry.  The  production  of  varieties  by  unit-character 
variation  is  nowhere  more  clearly  seen  than  among  domestic 
fowls.  The  wild  ancestor  is  supposed  to  be  represented  at 
present  in  the  jungle  fowl  of  India  {Gallus  bankiva)  a  small 
bird  of  bantam  size  having  the  color  character  of  the  breed 
known  as  brown  Leghorn,  and  producing  fully  fertile  off- 
spring in  crosses  with  domestic  breeds. 

Under  long  centuries  of  domestication  size  in  many  breeds 
has  been  increased,  though  certain  breeds  of  bantams  are  no 
larger  than  the  jungle  fowl.  Punnett  and  Bailey  (1914)  have 
maintained  that  several  unit  factors  are  concerned  in  size 
differences  between  bantam  and  ordinarv  breeds,  but  there 
is  some  doubt  as  to  the  correctness  of  their  interpretation. 
We  have  no  information  at  present  as  to  whether  the  bantam 
represents  the  persistent  small  size  of  the  wild  ancestor  or  has 
resulted  from  secondary  variation  in  races  of  normal  size. 
The  size  changes  from  the  wild  jungle  fowl  to  our  large 
breeds  of  poultry  have  undoubtedly  been  numerous  and 
probably  gradual,  involving  long-continued  selection. 

Color  variations  are  in  fowls,  as  among  mammals,  the  most 
conspicuous  unit-character  changes.  The  plumage  of  the 
jungle  fowl  contains  both  black  and  yellow  pigments  com- 
bined in  a  pattern  of  some  complexity.  This  pattern  may 
possibly  be  lost  or  suppressed  as  a  unit-character  variation, 
but  in  most  cases  it  is  changes  in  the  relative  amounts  of 
black  and  yellow  which  give  rise  to  self  black  or  self  yellow 
(red  or  buff)  breeds.  \ATiite  spotting  may  come  in  to  produce 
colorless  patches  in  the  plumage  and  if  these  become  suffi- 
ciently extensive  an  all-white  breed  results  such  as  the  white 
Leghorn.     The  white  of  Leghorns  is  a  dominant  character 

145 


146  GENETICS  AND  EUGENICS 

but  even  pure  bred  birds  may  develop  an  occasional  colored 
feather,  and  in  crosses  with  brown  Leghorns,  which  have  the 
ancestral  color,  the  heterozygotes  produced  may  show  traces 
of  color,  as  for  example  a  reddish  breast.  A  form  of  white 
plumage  genetically  distinct  from  the  foregoing  is  found  in 
white  silky  fowls  and  in  some  other  breeds.  In  this  the  down 
plumage  is  colored  and  the  adult  plumage  is  not  as  clear  and 
pure  a  white  as  that  of  white  Leghorns.  When  such  recessive 
whites  are  crossed  with  white  Leghorns,  fully  colored  offspring 
result  in  F2  though  not  in  Fi.  It  is  probable  that  recessive 
white  is  not  an  extreme  form  of  white  spotting,  as  perhaps 
the  white  of  Leghorns  is,  but  that  it  is  due  rather  to  some 
change  which  produces  fainter  pigmentation;  to  a  loss  varia- 
tion, rather  than  to  an  inhibition.  It  is  accordingly  com- 
parable with  the  albino  or  the  pink-eye  variation  of  rodents, 
whereas  the  white  of  Leghorns  is  comparable  with  the  black- 
eyed  white  variation  of  rodents,  an  extreme  form  of  white 
spotting.  Bateson  has  shown  that  there  are  two  or  possibly 
three  distinct  classes  of  recessive  white  varieties,  probably  of 
independent  origin,  for  when  two  of  these  (one  being  the 
white  silky)  were  crossed,  fully  colored  Fi  offspring  were  ob- 
tained similar  in  appearance  to  the  wild  Gallus  bankiva. 
This  is  a  result  comparable  with  that  obtained  when  pink- 
eyed  rodents  are  crossed  with  albinos  producing  fully  colored 
young.  It  shows  that  white  plumage  in  fowls,  like  pink  eyes 
and  pale  coats  in  rodents,  may  result  from  different  genetic 
changes.  Pigment  formation  is  a  complex  chemical  process 
in  which  several  factors  are  concerned.  Change  in  any  one 
of  these  may  interfere  with  the  normal  pigmentation. 

It  seems  doubtful  whether  the  Gallus  bankiva  pattern  is 
lost  in  the  ordinary  black  breeds  of  fowls;  more  probably  it 
is  simply  covered  up  by  an  excessive  development  of  black 
pigment.  Indeed  in  some  cases  the  pattern  is  faintly  visible 
in  the  black  breed  and  can  readily  be  brought  out  in  crosses. 
Such  varieties  are  comparable  with  the  blackened  agouti  va- 
rieties of  some  rodents  (black  squirrels  for  example) .  In  self 
yellow  (red  or  buff)  breeds,  the  pattern  fails  to  develop 


UNIT-CHARACTERS  OF  POULTRY  147 

^  merely  for  lack  of  black  pigment.  Yellow  varieties  are  im- 
perfectly recessive  to  black  in  crosses,  the  ancestral  pattern 
usually  resulting  in  Fi.  Blue  is  a  heterozygote  between  black 
and  splashed  white  (an  impure  sooty  strain  of  white).  It  is 
unfixable. 

A  color  pattern  of  fowls,  not  ancestral  in  origin,  but  domi- 
nant in  crosses  is  found  in  breeds  with  barred  plumage,  such 
as  the  Dominique  and  the  barred  Plymouth  Rock.  Its 
inheritance  is  sex-linked.  It  may  be  transmitted  through 
white  breeds,  as  for  example  the  white  Leghorn. 

A  black  pigmented  skin  associated  with  black  bones  is 
found  in  certain  strains  of  fowls,  e.  g,,  silkies.  This  is  domi- 
nant over  normal  (white  or  yellow)  skin. 

Several  morphological  variations  of  the  plumage  are  in- 
herited as  unit-characters.  Thus,  the  possession  of  a  topknot 
or  crest  (usually  associated  with  cranial  hernia)  is  an  im- 
perfectly dominant  character;  frizzled  (twisted)  feathers  are 
dominant  over  normal  feathers;  silky  feathers  (devoid  of 
barbules)  are  recessive  to  normal  feathers  (with  barbules). 
An  extra  or  fifth  toe  (due  to  a  divided  hind  toe)  is  an  imper- 
fectly dominant  character  found  in  Houdans  and  Dorkings. 
The  comb  is  also  a  highly  variable  character.  Single  comb  is 
the  form  found  in  Gallus  hankiva  and  in  the  connnoner  breeds 
of  poultry.  It  consists  of  a  high  serrated  ridge.  Pea  comb 
is  a  dominant  variation  from  this  ancestral  form  in  which  the 
comb  is  lower  and  broader,  without  distinct  serrations  but 
with  two  low  lateral  ridges  in  addition  to  a  chief  central 
ridge.  It  is  found  in  Indian  Games  and  the  Brahma  breeds. 
Rose  is  another  form  of  comb,  likewise  dominant  over  single. 
It  consists  of  a  broad  flat  comb  with  numerous  papillae  not 
arranged  in  distinct  rows.  A  cross  of  rose  with  pea  produces 
a  peculiar  type  of  comb  known  as  walnut,  which  is  found  in 
the  Malay  breeds.  When  produced  by  crossing,  it  does  not 
breed  true  without  fixation,  but  in  F2  gives  rise  to  walnut, 
rose,  pea,  and  single  comb  in  the  ratio,  9:3:3:1.  Evidently 
walnut  in  such  cases  is  due  to  the  joint  action  of  two  domi- 
nant factors  (R  and  P)  which  act  separately  in  pea-combed 


148 


GENETICS  AND  EUGENICS 


and  rose-combed  varieties  respectively,  and  when  both  P  and 
R  are  lacking  the  original  type  of  single  comb  is  formed. 


I 


TABLE  18 

Unit-Chaeacters  of  Domestic  Fowls 

A.   Sex-linked 

Dominant 

Recessive 

1. 

Black  skin  (silkies). 

Normal  skin  (dominant  in  femalcjs, 
imperfectly  recessive  in  males), 

2. 

Silver  (lacing,  spangling,  penciling). 

Gold  (lacing,  spangling,  penciling). 

3. 

Striped  down  of  chicks,  black  breast 
of  adult  male  (game  bantams). 

Plain  down,  brown  breast  of  male. 

4. 

Barred  feathers. 

Unbarred  feathers. 

4.( 

1  Spangling  (Hamburgs) 

Non-spangled  feathers. 

B.     Not  Sex-linked 

5. 

Black  plumage. 

Yellow  (or  buff  or  red)  plumage. 
(Heterozygote  often  like  jungle 
fowl.) 

6. 

White  of  white  Leghorns. 

Colored. 

7. 

Colored. 

\Miite  (of  silkies). 

8. 

Colored. 

\Miite  (of  rose-comb  bantams). 

9. 

Colored. 

"NMiite  (of  white  rocks). 

10. 

Normal  feathers. 

Silky  feathers. 

11. 

Frizzled  feathers. 

Plain  feathers. 

12. 

Crest. 

No  crest. 

13. 

Extra  toe. 

No  extra  toe. 

14. 

Yellow  skin. 

"VMiite  skin. 

15. 

Rmnpless. 

Normal  tail. 

16. 

Walnut  comb. 

Rose,  pea,  or  single  comb. 

17. 

Pea  comb. 

Single  comb. 

18. 

Rose  comb. 

Single  comb. 

19. 

Single  c(|jnib. 

Combless  (Breda). 

Plants.  No  attempt  will  be  made  at  a  detailed  survey  of 
unit-character  variations  in  plants  but  certain  general  cate- 
gories of  variations  may  be  indicated  and  examples  cited. 
These  will  serve  to  show  that  the  same  sorts  of  changes  are  at 
work  among  plants  as  among  animals  to  produce  striking 
varieties. 

1.  Colors  of  flowers.  Some  of  the  clearest  cases  relate  to  the 
colors  of  flowers.  Wild  species  often  exhibit  in  their  flowers 
a  mixture  of  pigments  associated  in  a  definite  pattern.  Loss 
or  suppression  of  the  pattern,  or  of  one  or  more  of  its  com- 
ponent colors,  leads  to  the  formation  of  self-colored  flowers. 


UNIT-CHARACTERS  OF  PLANTS  149 

or  those  which  are  white.  Thus  in  the  sweet  pea  the  wild 
plant  has  flowers  of  a  purple  bi-eolor,  resulting  from  the  asso- 
ciation of  red  and  blue  pigments  in  a  definite  pattern.  Red 
flowers  may  arise  by  a  suppression  of  a  factor  for  blue.  This 
change  alone  produces  a  red  flower  with  wings  lighter  than 
the  standard  (a  red  bi-color).  Another  recessive  factorial 
change  does  away  with  the  lightness  of  the  wings,  producing 
a  flower  with  both  wings  and  standard  full  red.  A  corre- 
sponding change  in  pattern  in  purple  (the  original  color),  not 
attended  by  suppression  of  blue,  produces  purple  with  both 
wings  and  standard  of  full  color.  A  quantitative  change  in 
the  color  factor  (a  partial  loss  of  color)  produces  faintly 
colored  varieties  known  as  picotee,  either  purple  or  red.  In 
the  flowers  of  many  cultivated  plants  striping,  mottling  or 
spotting  with  white  or  red  comes  in  as  a  unit-character  varia- 
tion, as  in  petunias,  snapdragons,  etc. 

4, 

TABLE  19 

Unit-Chakacters  of  Plants 

1.  Colors  of  Flowers 
(Example,  unit-characters  of  the  sweet  pea  flower.) 

Dominant  Recessive 

(1)  Colored.  White. 

(2)  Colored.  Slightly  colored  (picotee). 

(3)  Purple.  Red. 

(4)  Bi-color.  Self. 

2.  Forms  of  Flowers 

(1)  Normal.  Peloric. 

(2)  Single.  Double. 

3.  Colors  of  Leaves  and  Stem 

(1)  Variegated  with  yellow.  Normal  green  (dominance  imperfect). 

(2)  Containing  much  red.  With  little  red  (Oenothera,  Coleus,  maize). 

4.  Colors  of  Fruits  and  Seeds 

(Example,  maize) 

(1)  Yellow  endosperm.  White  endosperm. 

(2)  Aleurone  black.  Aleurone  red  or  uncolored. 

(3)  Aleurone  red.  Aleurone  uncolored, 

(4)  Endosperm  starchy.  Endosperm  sugary. 

(5)  Endosperm  starchy.  Endosperm  waxy. 

-  (6)  Seed-coat  red.  Seed-coat  colorless. 

(7)  Seed-coat  variegated.  Seed-coat  not  variegated. 


150  GENETICS  AND  EUGENICS 

5.  Forms  of  Leaves 

(1)  Serrate.  Entire  (Urtica,  Fig.  115). 

(2)  Normal.  Laciniate  (Chelidonium). 

(3)  Palmate.  Pinnatifid  or  fern-leaf  (Primula). 

(4)  Hairy.  Glabrous  (dominance  often  imperfect). 

2.  Forms  of  flowers.  The  forms  of  flowers,  no  less  than 
their  colors,  are  subject  to  unit-character  variation.  In 
sweet  peas  the  ordinary  form  of  flower  with  erect  standard  is 
dominant  over  a  variation  in  which  the  standard  lops  down 
at  either  corner  forming  what  is  called  a  "hood."  Symmetri- 
cal forms  of  flowers  which  appear  as  sports  in  species  having 
normally  asymmetrical  flowers  are  a  unit-character  variation. 
Thus  a  peloric  (symmetrical)  variation  in  the  snapdragon  is 
recessive  to  normal  (asymmetrical)  shape  of  flower  (Baur). 
Double  flowers,  those  which  have  an  increased  number  of 
parts  (commonly  petals),  are  in  general  recessive  to  singles. 
This  is  the  case  for  example  in  primulas,  poppies  and  lark- 
spurs. But  some  cases  occur  in  which  the  heterozygote  is 
intermediate,  as  for  example  in  carnations.  Here  a  good 
commercial  double  type  is  found  to  be  regularly  heterozy- 
gous, producing  when  selfed  both  singles  and  extremely 
double  types  (**busters"),  each  of  which  sorts  breeds  true, 
and  in  addition  the  unstable  but  more  valuable  heterozygous 
type  of  the  parent  (Norton) . 

3.  Colors  of  leaves  and  stems.  The  colors  of  leaf  and  stem 
often  vary  abruptly  in  cultivated  plants  by  unit-character 
changes.  Thus  strains  variegated  with  yellow  arise  from 
local  loss  or  inhibition  of  chlorophyl,  a  change  which  impairs 
the  assimilative  power  of  the  plant  but  adds  to  its  ornamental 
value  in  horticulture.  Of  course  plants  largely  or  completely 
yellow  because  of  deficiency  of  chlorophyl  would  be  unable 
to  maintain  themselves  other  than  as  parasites,  such  as 
dodder;  hence  the  yellow  of  variegated  plants  is  usually 
Hmited  in  amount.  Some  varieties  of  cultivated  plants  pos- 
sess as  a  distinguishing  character  an  unusual  amount  of  red 
coloring  matter  (anthocyan)  in  leaf  or  stem.  Examples  of 
this  are  seen  in  purple  beeches  and  maples,  variations  known 


UNIT-CHARACTERS  OF  PLANTS 


151 


to  have  originated  as  sports  and  doubtless  Mendelizing  in 
crosses.  The  cultivated  celosias  are  good  examples  of  i)lants 
in  which  an  excessive  amount  of  anthocyan  pigment  pro- 
duces brilliant  red  or  yellow  plants,  the  latter  a  probably 
recessive  sport  from  the  former,  just  as  the  yellow  fruit  of 
the  tomato  is  known  to  be  recessive  to  red  fruit.    In  Coleus 

mica 
DoddTttU/iilulifera 


jiilulifeTA 


Dodartii 


.'4  4 


41  HU  *il4  II 


ten. 

Fig.  115.  A  Mendelian  cross  between  two  varieties  of  nettle  differing  in  shape  of  leaf,  I.  Gen.  =  Fi. 
II.  Gen.  =  F2.  III.  Gen.  =  F3.  The  diagram  indicates  that  the  serrated  form  is  dominant,  the  re- 
cessive form  reappearing  in  F2  and  breeding  true  in  F.3.    (After  Correns.) 

the  red  has  a  mosaic  and  highly  variable  distribution  on  the 
green  leaves,  like  that  of  yellow  spotting  in  mammals. 

4.  Colors  of  fruits  and  seeds.  The  colors  of  fruits  and  seeds 
vary  discontinuously  in  the  same  way  that  the  colors  of 
flowers,  leaves  and  stems  vary.  As  an  example  we  ma}^  con- 
sider some  variations  in  the  color  and  composition  of  the 
seed  of  maize.  The  common  varieties  of  corn  are  either 
yellow  or  white  seeded,  the  yellow  grain  containing  a  yellow 
colored  endosperm,  a  character  dominant  to  white.  A  black 
pigment  which  is  present  in  the  aleurone  layer  just  under  the 
seed-coat  is  responsible  for  a  dominant  variation  in  some 


152  GENETICS  AND  EUGENICS 

varieties.  Red  aleurone  color  is  a  recessive  allelomorph  of 
black.  Both  are  dominant  over  colorless  aleurone.  Red 
seed-coat  is  a  character  dominant  over  colorless  seed-coat, 
and  a  seed-coat  striped  with  red  is  allelomorphic  to  unstriped 
seed-coat.  A  highly  starchy  condition  of  the  endosperm  is 
found  in  ordinary  varieties  of  field  corn,  which  have  rela- 
tively plump  seeds.  A  recessive  allelomorphic  condition  is 
found  in  sweet  corn  cultivated  for  table  use,  in  which  sugar 
predominates  in  the  seeds  so  that  on  drying  it  takes  on  a 
shriveled,  wrinkled  appearance.  A  different  recessive  varia- 
tion is  found  in  a  variety  of  corn  recently  imported  from 
China,  in  which  the  endosperm  is  waxy  rather  than  sweet  or 
starchy.  If  the  variety  with  waxy  endosperm  is  crossed  with 
sweet  corn,  starchy  corn  is  obtained  by  reversion  in  Fi,  and 
in  F  2  all  three  sorts  are  obtained  in  the  ratio,  nine  starchy 
to  three  waxy,  and  four  sweet. 

5.  Forms  of  leaves.  Leaf  form  in  many  cultivated  plants 
is  known  to  vary  by  Mendelizing  units.  In  the  nettle 
(Urtica)  Correns  has  shown  that  the  much-serrated  leaves  of 
one  natural  variety  possess  a  character  dominant  over  the 
nearly  entire  leaves  of  another  variety  (Fig.  115).  In  Cheli- 
donium  majus,  sl  laciniate  leaf  form  is  known  to  be  recessive 
to  the  normal  form  of  leaf.  In  Primula  sinensis,  normal  pal- 
mate leaves  are  dominant  over  fern -like  pinnatifid  leaves.  In 
a  great  number  of  plants  hairy  or  spinous  leaves,  stems,  or 
fruits,  are  known  to  be  dominant  (more  or  less  completely) 
over  smooth  ones. 

6.  Form  of  stem.  One  of  the  seven  discontinuous  varia- 
tions with  which  Mendel  dealt  in  his  original  paper  is  in- 
volved in  the  difference  between  tall  and  dwarf  races  of  peas 
and  beans.  The  original  and  the  dominant  form  of  stem  is 
the  tall  form.  Dwarf  form,  in  which  the  intemodes  of  the 
plant  are  relatively  short,  segregates  in  regular  recessive 
fashion.  Semi-dwarf  races  also  exist,  which  indicate  either 
imperfect  segregation  or  alternative  forms  of  dwarfness. 
Dwarfness  occurs  as  a  variation  alternative  to  normal  tall 
form  in  snapdragons,  nasturtiums,  and  many  other  culti- 
vated plants. 


UNIT-CHARACTERS  OF  PLANTS  153 

The  original  much-branched  condition  of  the  annual  sun- 
flower and  of  stocks  and  of  many  other  cultivated  plants  is 
dominant  over  the  unbranched  condition  found  in  certain 
cultivated  races. 

These  illustrations  serve  to  show  that  practically  all  parts 
and  structures  of  plants,  as  well  as  of  animals,  are  likely  to  be 
affected  by  unit-character  variations  and  that  combining  of 
such  variations  by  means  of  crossing  is  a  ready  means  of 
producing  new  varieties. 


CHAPTER  XVI 

UNIT-CHARACTERS  OF  INSECTS 

The  so-called  "  silkworm  "  is  the  larva  of  an  Asiatic  moth 
which  feeds  principally  on  the  leaves  of  the  mulberry  tree. 
The  "  worms  "  when  full  grown  spin  a  silken  cocoon  (which 
furnishes  the  silk  of  commerce)  within  which  they  complete 
their  metamorphosis  into  the  moth  stage.  As  moths  they 
mate  and  the  females  lay  eggs.  In  some  races  there  is  only 
one  generation  a  year,  the  eggs  laid  one  summer  hatching  the 
next  spring.  These  are  said  to  be  univoltine,  having  one 
flight  or  mating  period  annually.  In  other  races  there  are 
two  or  more  broods  a  year  depending  on  temperature  condi- 
tions. These  are  said  to  be  bivoltine  or  multivoltine.  In 
crosses  between  univoltine  and  bivoltine  races  the  eggs  laid 
have  the  character  of  the  mother's  race,  being  purely  ma- 
ternal structures.  Thus,  eggs  laid  by  a  univoltine  mother 
refuse  to  hatch  before  the  following  season,  whatever  the 
racial  character  of  the  male  that  fertilized  the  eggs.  And 
eggs  laid  by  a  bivoltine  mother  are  regularly  bivoltine  regard- 
less of  the  father's  racial  character.  But  the  females  which 
hatch  from  cross-bred  eggs  are  really  heterozygous  as  regards 
voltinism.  Their  eggs  show  the  dominant  {univoltine)  char- 
acter but  their  daughters,  the  F2  females,  are  some  univol- 
tine, others  bivoltine,  in  the  ratio,  3:1. 

Races  of  silkmoths  differ  by  numerous  characters,  many 
of  which  are  Mendelian.  Toyama  has  enumerated  more 
than  a  dozen  such  Mendelizing  characters  found  in  the  larva 
alone.  Some  races  differ  in  the  number  of  larval  moults, 
which  may  be  either  three  or  four.  Tri-moulting  is  dominant 
over  tetra-moulting  in  crosses.  The  blood  of  the  larva  may 
or  may  not  be  yellow  colored,  yellow  blood  being  dominant. 
Yellow-blooded  larvae  spin  yellow  cocoons  so  that  there  is  a 
correlation  between  blood-color  of  the  larva  and  the  cocoon- 

154 


UNIT-CHARACTERS  OF  INSECTS  155 

color.  Presence  of  pigments  in  the  larval  skin  is  dominant 
over  uncolored  skin.  Various  patterns  of  the  larval  pigmen- 
tation (spotting,  striping,  etc.)  are  dominant  over  their  alj- 
sence.  Reddish-brown  color  of  the  larva  is  recessive  to  bhick. 
The  possession  of  knob-like  outgrowths  of  the  larval  skin  is 
dominant  over  smooth  skin. 

TABLE  20 

Unit-Characters  of  Silkworms 

1.  Egg  Characters,  all  Maternal  in  Origin 
Dominant  Recessive 

(1)  TJnivoltine.  Bivoltine. 

(2)  Eggs  oval.  Eggs  spindle-shape. 

(3)  Eggs  normal  slate  color.  Eggs  light  brown  or  gray. 

2.  Characters  of  the  Larva  or  the  Cocoon,  of  Biparental  Origin 

(1)  Tri-moulting.  Tetra-moulting. 

(2)  Blood  (and  silk)  yellow.^  Blood  (and  silk)  white. 

(3)  Silk  white  (European  races) .  Silk  yellow. 

(4)  Larval  skin  pigmented.  Larval  skin  unpigmented. 

(5)  Larva  spotted  or  striped.  Larva  not  spotted  or  striped. 

(6)  Larva  black.  Larva  reddish  brown. 

(7)  Larval  skin  with  knob-like  Larva  not  knobby. 

outgrowths. 

White  cocoon-color  (silk)  has  been  found  in  some  races  to 
be  a  recessive  character  and  in  others  to  be  dominant.  The 
two  kinds  of  white  have  been  shown  to  be  genetically  distinct. 
One  is  probably  a  loss  variation  like  albinism  in  rodents,  the 
other  a  white  variation  due  to  inhibition  of  color,  like  some 
forms  of  white  spotting  in  mammals.  Certain  variations  in 
the  color  and  shape  of  the  egg  have  been  found  to  Mendelize, 
but  with  the  same  complication  as  in  the  variation  from 
univoltinism  to  bivoltinism.  Egg  characters  being  deter- 
mined entirely  by  the  mother,  the  influence  of  the  father  does 
not  show  in  the  Fi  generation.  Which  of  the  contrasted 
characters  is  dominant  does  not  become  evident  until  eggs 

^  Uda  has  recently  shown  that  yellow  color  of  the  blood  is  due  to  a  single  domi- 
nant factor  but  that  the  silk  will  also  be  yellow  only  when  a  second  and  independent 
factor  is  also  present.  When  this  second  factor  is  lacking,  white  silk  will  appear  to 
be  dominant  over  yellow  silk,  even  though  the  blood  is  yellow. 


156  GENETICS  AND  EUGENICS 

are  laid  by  the  Fi  females  and  segregation  is  seen  first  in  the 
eggs  laid  by  F2  females.  Spindle-shape  of  egg  is  a  recessive 
variant  from  normal,  oval  shape,  and  light  brown  egg-color 
and  gray  egg-color  are  recessive  variations  from  normal 
slate-color. 

Bateson  (1913)  has  brought  together  records  for  numerous 
cases  of  unit-character  color  variation  in  moths  and  beetles 
occurring  in  the  wild  state.  These  cases  present  nothing  in 
principle  different  from  the  variations  of  silkworms,  but 
show  that  Mendelian  sports  occur  among  insects  "in  nature" 
as  well  as  under  artificial  conditions. 

The  most  complete  and  in  many  respects  the  most  instruc- 
tive series  of  unit-character  variations  recorded  in  any  insect 
has  taken  place  within  a  very  few  years  in  a  small  fruit  fly, 
Drosophila,  while  it  was  under  observation  in  the  Zoological 
Laboratory  of  Columbia  University.  For  this  discovery  we 
are  indebted  to  Professor  T.  H.  Morgan  and  his  pupils. 
Drosophila  melanog aster  is  a  small  fly  with  grayish  brown 
body  and  red  eyes,  which  lays  its  eggs  in  fermenting  fruits. 
Apples,  peaches,  grapes  or  bananas  with  broken  skin  afford 
good  conditions  for  its  multiplication.  It  is  sometimes  known 
as  the  vinegar  or  pomace  fly  because  the  alcoholic  fermenta- 
tion of  apple  juice  attracts  it  to  vinegar  jugs,  pickle  jars,  and 
cider  mills.  This  fly  while  breeding  in  Professor  Morgan's 
laboratory  produced  a  white-eyed  sport,  which  lacked  en- 
tirely the  normal  red  eye-color.  The  sport  was  first  observed 
in  a  male  individual,  which  bred  to  normal  mates  produced 
only  normal  offspring.  But  when  these  Fi  offspring  were 
bred  together  they  produced  white-eyed  offspring  as  reces- 
sives  in  the  expected  proportion,  one-fourth.  Curiously 
enough,  however,  all  were  males.  Nevertheless,  when  these 
obviously  recessive  white-eyed  males  were  mated  with  Fi 
females  (heterozygotes)  a  generation  was  produced  consisting 
of  white-eyed  individuals  and  red-eyed  individuals  in  equal 
numbers,  and  among  both  sorts  the  sexes  were  approximately 
equal.  White-eyed  individuals  bred  together  breed  true,  but 
in  crosses  the  white-eyed  character  seems  to  have  a  prefer- 


UNIT-CHARACTERS  OF  INSECTS  157 

ence  for  male  individuals,  which  has  led  to  its  being  called  a 
sex-linked  character.  White-eye  has  proved  to  be  only  the 
first  of  a  long  series  of  unit-character  variations,  which  have 
appeared  in  Professor  Morgan's  cultures  of  Drosophila, 
which  have  this  same  curious  sex-linked  character.  Among 
these  may  be  mentioned  a  variation  in  which  the  entire  body 
is  yellow,  another  in  which  the  eye-color  instead  of  being  an 
ordinary  red,  is  a  brilliant  vermilion,  and  several  variations  in 
the  form  of  the  wing  known  as  rudimentary,  miniature, 
forked,  etc.  It  is  found  that  when  a  race  possessing  two  of 
these  recessive  sex-linked  characters  (as  white  eye  and  yellow 
body)  is  crossed  with  another  race  which  lacks  them,  there  is 
a  tendency  for  the  two  sex-linked  characters  to  go  together 
in  heredity,  so  that  whatever  F2  individuals  possess  one  of 
them  possess  also  the  other.  This  suggests  that  the  material 
basis  or  '*gene''  of  each  lies  in  the  germ-cell  near  that  of  the 
other,  that  their  genes  are  either  connected  directly  with  each 
other  or  with  a  common  third  structure.  Since  there  are 
several  of  these  variations  which  show  ''linkage"  with  each 
other  and  a  peculiar  relationship  to  sex,  the  pertinent  sug- 
gestion was  made  by  Morgan  that  they  had  as  a  common 
connecting  element  a  structure  concerned  in  the  determina- 
tion of  sex,  commonly  known  as  the  sex-chromose  or  X-chro- 
mosome.  The  **genes"  of  sex-linked  characters,  according 
to  Morgan,  lie  in  the  X-chromosome  and  the  peculiar  features 
of  the  inheritance  are  due  to  the  fact  that  the  X-chromosome 
is  paired  in  females  but  unpaired  in  males.  Strong  support 
is  given  to  this  idea  by  the  result  of  crosses  in  which  each 
parent  introduces  a  different  sex-linked  character,  as  in  the 
cross  between  a  white-eyed  race  and  a  yellow-bodied  race, 
each  being  otherwise  normal.  The  two  characters  in  this 
case  keep  apart  as  strongly  as  they  keep  together  when  in- 
troduced into  a  cross  by  the  same  parent.  This  is  exactly 
what  we  should  expect  if,  as  Morgan  supposes,  sex-linked 
characters  have  their  genes  in  a  common  cell  structure  (for 
example  an  X-chromosome).  For  when  two  genes  lie  in  the 
same  X-chromosome,  they  will  go  together  (show  linkage). 


158  GENETICS  AND  EUGENICS 

but  when  they  lie  in  different  X-chromosomes,  as  for  example, 
in  those  furnished  by  the  father  and  mother  respectively, 
then  each  will  go  with  a  different  X,  when  the  paired  chromo- 
somes separate  from  each  other,  as  they  do  when  gametes  are 
formed. 

But  we  are  forced  to  suppose  that  occasionally  in  the  eggs 
of  Drosophila  a  gene  may  detach  itseff  from  one  X-chromo- 
some  and  pass  over  into  the  other,  for  once  in  a  while  we  find 
that  two  sex-linked  characters  which  were  repelling  each 
other  have  in  some  way  got  into  the  same  gamete  and  are 
now  coupled,  and  vice  versa  two  which  were  coupled  may  later 
show  repulsion.  Morgan's  hypothesis  offers  a  simple  expla- 
nation of  such  occurrences.  The  supposed  changing  of  a  gene 
from  one  X-chromosome  to  another,  when  repulsion  gives 
place  to  coupling  or  vice  versa,  Morgan  calls  a  **  crossing- 
over."  It  occurs  only  in  female  individuals,  or  more  properly 
in  their  eggs,  for  it  has  not  been  observed  to  occur  in  the 
sperms  of  Drosophila. 


CHAPTER  XVII 

SEX-LINKED  AND  OTHER  KINDS  OF  LINKED  INHERITANCE 

IN  DROSOPHILA 

All  the  facts  of  sex-linked  inheritance  in  Drosophila  liar- 
monize  with  Morgan's  hypothesis  that  the  genes  of  sex-linked 
characters  lie  in  a  common  cell  structure  (X-chromosome) 
which  is  duplex  in  females,  simplex  in  males.  Accordingly, 
in  a  race  which  breeds  true  for  a  sex-linked  character,  that 
character  may  be  transmitted  by  every  egg,  but  by  only  half 
the  sperms,  namely  by  such  as  possess  an  X-chromosome  and 
by  virtue  of  that  fact  determine  as  female  all  zygotes  into 
which  they  enter.  To  male  zygotes  the  sperm  will  not  trans- 
mit sex-linked  characters.  This  hypothesis  is  supported  by 
some  curious  facts  already  alluded  to  but  deserving  of  fuller 
consideration  in  this  connection,  viz.,  facts  observed  in  re- 
ciprocal crosses  involving  a  sex-linked  character,  as  for 
example  white  eye  in  Drosophila. 

TABLE  21 

Reciprocal  Crosses  of  White-Eyed  with  Red-Eyed  Drosophila 

Male  Female  Male  Female 

P  White      X       Red  Red  X  White 

Fi  Red  Red  White  Red 

F2       1  Red  :1  White        Red  1  Red:  1  White        1  Red  :1  White 

It  has  already  been  stated  that  a  white-eyed  male  Droso- 
phila crossed  with  normal  females  has  only  normal  children 
of  both  sexes,  while  the  white-eyed  grandchildren  are  all  of 
the  male  sex.  In  the  reciprocal  cross,  between  a  white-eyed 
female  and  a  normal  male  all  the  daughters  are  normal,  but 
the  sons  are  white-eyed,  and  among  the  grandchildren  white- 
eyed  individuals  occur  in  both  sexes.  Diagrams  will  best 
explain  these  facts  on  the  basis  of  Morgan's  hypothesis. 
(See  Figs.  116  and  117  and  Table  21.) 

159 


160 


GENETICS  AND  EUGENICS 


To  state  the  foregoing  facts  in  another  way,  it  will  be  ob- 
served that  the  recessive  sex-linked  character  in  Drosophila, 
when  introduced  in  a  cross  by  the  male  parent,  disappears 
entirely  in  Fi  and  reappears  in  F2  only  in  male  individuals. 


Flies 


(S 


Chromosomes 


XX 

IXI 
X     X    $ 


2YO:  2X11 


X  ® 
iXi 
X     1 


XX  XI  X 


Parents 


Gametes 


Fi 


Gametes 


Fz 


Fig.  116.  Sex-linked  inheritance  of  white  and  of  red  eyes  in  Drosophila.  Parents,  white-eyed  male  and 
red-eyed  female;  Fi,  red-eyed  males  and  females;  F2,  red-eyed  females  and  equal  numbers  of  red-eyed 
and  white-eyed  males.  A  black  X  indicates  an  X-chromosome  bearing  the  gene  for  red  eye,  a  white 
X  bears  white  eye.  O  indicates  that  an  X  is  wanting;  in  recent  publications  Morgan  replaces  it  by  Y. 
(From  Conklin,  after  Morgan.) 

But  if  the  recessive  sex-linked  character  is  introduced  by  the 
female  parent,  it  appears  in  Fi  in  male  individuals  but  in  F2 
in  both  sexes. 

Suppose  now  a  cross  is  made  between  two  races,  each  of 
which  possesses  a  different  sex-linked  recessive  character,  as 
for  example  white  eye  and  yellow  body.  (See  Table  22.)  If 
the  white-eyed  parent  is  a  female,  there  will  be  produced 
white-eyed  males  in  Fi  and  white-eyed  flies  of  both  sexes  in 
F2.  But  the  male  parent  being  yellow,  there  will  be  no  yellow 
flies  produced  in  Fi  and  only  yellow  males  in  F2.  In  the  re- 
ciprocal cross   (yellow  female  X  white-eyed   male)   yellow 


SEX-LINKED  INHERITANCE  IN  DROSOPHILA  161 

males  will  be  produced  in  Fi  and  yellow  flies  of  both  sexes  in 
F2,  while  white-eyed  flies  will  not  appear  until  F2  and  then 
only  in  the  male  sex.  In  either  of  the  reciprocal  crosses  we 
expect  the  production  in  F2  both  of  yellow-bodied  males  and 


Flies 


Chromosomes 


X0 

6 


iXi 


Parents 


Gametes 


<S  ? 

X    IX     ? 


Fi 


Gametes 


M  M  X0  m  Fz 


Fig.  117.  Reciprocal  cross  to  that  shown  in  Fig.  116.  Parents,  red-eyed  male  and  white-eyed  female; 
Fi,  white-eyed  males  and  red-eyed  females  ("  criss-cross  inheritance  "  —  Morgan);  Fs,  equal  numbers 
of  red-eyed  and  white-eyed  individuals  in  both  sexes.  The  distribution  of  the  sex-chromosomes  is 
shown  at  the  right,  as  in  Fig.  116,    (From  Conklin,  after  Morgan.) 

of  white-eyed  males.  Usually  no  other  sort  of  male  is  pro- 
duced throughout  the  experiment  except  these  two,  but  occa- 
sionally there  is  produced  a  male  both  yellow-bodied  and 
white-eyed,  or  one  which  is  gray-bodied  and  red-eyed,  like 
wild  flies.  How  do  these  arise  ?  If  in  Fi  females  the  paired 
X's  were  to  exchange  loads  in  part,  so  that  G  and  R  came 
to  be  attached  to  the  same  X  and  g  and  r  to  the  other  X, 
and  if  each  of  the  eggs  having  such  a  constitution  were  to  be 
fertilized  with  a  sperm  which  lacked  X  (male  determining 
sperm),  this  would  make  possible  the  production  of  Fo  males 
possessing  both  dominant  characters  and  others  possessing 
both    recessive    characters    or   gray-red    and    yellow-white 


162  GENETICS  AND  EUGENICS 

respectively,  as  actually  observed  in  about  one  case  in  a  hun- 
dred by  Morgan. 

It  may  add  interest  to  the  case  to  state  parenthetically  that 
in  man  occur  a  number  of  sex-linked  variations  which  are  in- 
herited in  this  same  curious  fashion.  Among  them  may  be 
mentioned  color  blindness  and  bleeding  {haemophilia),  which 

TABLE   22 
Reciprocal  Crosses  of  White-Eyed  and  Yellow-Bodied  Flies 

Male  Female  Male  Female 

P        Yellow-red       X  Gray-white  Gray-white      X  Yellow-red 

Fi       Gray-white  Gray-red  Yellow-red  Gray-red 

F2       1  Gray-white:  1  Gray-red:  1  Gray-white:  1  Gray-red: 

1  Yellow-red  1  Gray-white  1  Yellow-red  1  Yellow-red 

occur  chiefly  in  males,  but  are  never  transmitted  by  males  to 
their  sons  but  only  through  their  daughters  to  their  grand- 
sons. 

Morgan  and  his  pupils  have  described  between  forty  and 
fifty  characters  in  Drosophila  which  are  sex-linked  in  hered- 
ity; they  also  have  discovered  a  large  number  of  other 
Mendelizing  characters  in  Drosophila  which  are  not  sex- 
linked  but  which  nevertheless  are  inherited  in  groups,  char- 


$?,6 


Fig.  118.    Drawing  showing  the  four  pairs  of  chromosomes  seen  in  the 
dividing  egg-ceil  of  Drosophila.     (After  Dr.  C.  W.  Metz.) 

acters  in  the  same  group  showing  coupling  when  introduced 
in  a  cross  from  the  same  parent,  and  repulsion  when  intro- 
duced from  different  parents.  The  number  of  these  groups 
exactly  corresponds  with  the  number  of  the  chromosomes  and 
Morgan  believes  that  their  genes  are  located  in  the  chromo- 
somes, an  hypothesis  which  seems  reasonable  but  which 
would  be  severely  strained  if  an  additional  group  of  characters 
should  be  discovered.  There  are  three  groups  of  the  non-sex- 
linked  characters.  (See  Fig.  119.)  In  one  of  these  referred 
to  as  Group  II  (the  sex-linked  group  being  called  Group  I), 


.oo  YELLOW,  SPOT. 
:0.7  LETHAL  I. 
»-o  W  IIITE.  EOSIN,  COEBHY. 

»o  ABNORMAL. 


••O  BIEID. 


•0.0  STREAK. 


•0.0  SEPIA. 


•  14.7  CLITB. 


■!•.•  SHIFTED. 


•  20.5  LETHAL  nr. 

•27.3  TAN. 


■BENT. 


•ETELES8. 


■!•.•  DA  CHS. 


•  a»«  PINIC  PEACH. 


■33.0  VERMIUQH. 
•3e.a  MINIATURE. 


•9*.7  BLACK. 


■41,7  LETHAL  V. 
■43.0  SABLE. 


■40.3  LETHAL  IV. 


-40.«  PURPLE. 


-40.  KIDNEY. 


-•a.o  VESTIGUJt* 


■8B.1  RUDIMENTARY. 

»«-8  FORKED. 
"87.0  BARRED. 


.EBONY,  SOOTTj 


•  SO.S  FUSED. 


.  ©0.4  CURVED. 


■08.9  LETHAL  S. 


Fig.  119.   Diagram  show- 
ing the  location,   in   the 
four  paired  chromosomes 
of     Drosophila,    of     the 
genes    for   various   Men- 
delizing  characters,  as  de- 
termined by  Morgan  and 
his  pupils.     The  X-chro- 
mosome   is   at    the   left. 
All  characters  there  enu- 
•  merated    are    sex-linked. 
The     numerals    indicate 
the  supposed  relative  dis- 
tances of  the  genes  from 
the  upper  (zero)  end  of 
each  chromosome  as  de- 
termined   by    linkage 
strengths  in  crosses.    (Af- 
\%^  Morgan,  Sturtevant, 
Muller  and  Bridges.) 


■  7».  BEADED. 


■•44  ARC 


>  80.0  SPECK. 
•«.©  MORULA. 


-•f.OTlOUGH 


[\ 


( 


\ 


SEX-LINKED  INHERITANCE  IN  DROSOPHILA    163 

are  found  variations  knowTi  as  black  body  and  vestigial  wings 
respectively,  together  with  some  thirty-five  other  variations. 
In  Group  III  are  found  the  variations  known  as  pitik  eye, 
spread  wings,  and  ebony  body,  together  with  some  twenty 
other  variations.  In  Group  IV  are  included  as  yet  only 
two  characters,  bent  wings  and  eyeless,  which  however  show 
linkage  with  each  other.  No  inherited  characters  have  been 
discovered  in  Drosophila  which  are  not  inherited  in  one  or 
another  of  the  four  linkage  groups. 


CHAPTER  XVIII 

DROSOPHILA  TYPE  AND  POULTRY  TYPE  OF  SEX-LINKED 

INHERITANCE 

1.  Drosophila  type.  The  same  type  of  sex-linked  inheri- 
tance which  is  found  in  Drosophila  is  found  also  in  man,  in 
cats  (inheritance  of  yellow  color),  and  in  the  plants,  Lychnis 


Fig.  120.  Sex-linked  inheritance  of  barred  and  of  unbarred  (black)  plumage  in  poultry.  P,  parents, 
barred  male,  unbarred  female;  Fi,  barred  males  and  females;  F2,  males  all  barred,  females  in  equal 
numbers  barred  and  unbarred.     (After  Morgan.) 

and  Bryonia.  The  essential  feature  of  this  "  Drosophila 
type  "  of  inheritance  is  this.  In  a  race  breeding  true  for  a 
sex-linked  character,  the  female  is  homozygous  for  the 
character  in  question  while  the  male  is  heterozygous  and  in- 

164 


SEX-LINKED  INHERITANCE  IN  POULTRY     165 

capable  of  becoming  homozygous.  Reciprocal  crosses  with 
such  a  race  give  unlike  results,  because  the  female  transmits 
the  character  to  all  her  offspring,  but  the  male  transmits  it 
to  only  half  his  offspring,  viz.,  the  females. 

2.  Poultry  type.  Another  type  of  sex-linked  inheritance 
exists  in  which  the  sex  relations  are  exactly  reversed.  This 
was  first  observed  in  the  moth.  Abraxas,  but  more  familiar 
cases  occur  in  poultry,  for  which  reason  it  may  be  called  the 
poultry  type  of  sex-linked  inheritance.    Here  the  male  is  the 


Fig.  121.  Reciprocal  cross  to  that  shown  in  Fig.  120.  F,  parents,  unbarred  male,  barred  female;  Fi, 
barred  males,  unbarred  females  (criss-cross  inheritance);  F2,  barred  and  unbarred  birds  equally  nu- 
merous in  both  sexes. 

homozygous  sex,  the  female  being  heterozygous.  This  condi- 
tion is  found  in  moths  and  in  certain  birds,  viz.,  m  domestic 
fowls,  pigeons,  ducks  and  canaries.  As  an  example  we  may 
take  the  inheritance  of  the  color  pattern,  barring,  in  crosses  of 


166  GENETICS  AND  EUGENICS 

barred  Plymouth  Rock  fowls.  In  reciprocal  crosses  between 
pure-bred  barred  Plymouth  Rocks  and  black  Langshans 
(or  any  other  unbarred  breed),  the  results  are  not  identical.  If 


TABLE  23 

Reciprocal  Crosses  of  Barred  and  Black  Breeds 

\  OF  Fowls 

Male                        Female                                        Male 

Female 

p 

Barred   X   Black                                  Black             X 

Barred 

Fi 

Barred          Barred                                  Barred 

Black 

F2 

Barred          1  Barred:  1  black             1  Barred:   1  black 

1  Barred:   1  black 

See  Fig.  120.                                                    See  Fig 

121. 

the  barred  parent  is  the  male  (Fig.  120  and  Table  23),  all  Fi, 
offspring  are  barred  and  in  F2  all  males  are  barred,  but  half  the 
females  are  black  and  half  are  barred.  If,  however,  the  barred 
parent  is  the  female  (Fig.  121  and  Table  23),  all  Fi  males  are 
barred,  but  all  Fi  females  are  black.  In  F2  barred  birds  and 
black  birds  occur  in  both  sexes.  These  curious  facts,  which 
have  been  repeatedly  verified,  suggest  the  occurrence  of  a 
vehicle  of  inheritance  which  is  duplex  in  males  but  simplex  in 
females.  What  this  is  we  do  not  know.  No  chromosome  has 
been  found  which  has  a  distribution  of  this  sort  in  fowls,  but 
it  is  possible  that  some  chromosome  component,  or  other  cell 
constituent,  has  such  a  distribution  and  may  be  the  actual 
vehicle  of  inheritance  in  such  cases.  The  most  important 
character  economically,  which  appears  to  be  affected  by  some 
sex-linked  factor  in  poultry,  is  fecundity.  Pearl  has  observed 
that  when  reciprocal  crosses  are  made  between  Cornish  In- 
dian games,  a  poor  breed  for  winter  egg  production,  and 
barred  Plymouth  Rocks,  a  fairly  good  breed  for  winter  egg 
production,  the  Fi  females  in  each  case  resemble  the  father's 
race  more  strongly  than  the  mother's  race  as  regards  egg 
production.  Pearl  did  not  maintain,  however,  nor  do  his  ex- 
periments suggest,  that  the  inheritance  of  fecundity  depends 
exclusively  upon  a  sex-linked  factor.  Goodale,  however,  has 
not  been  able  to  confirm  Pearl's  observations,  in  the  case  of 
Rhode  Island  Red  fowls.  He  finds  no  evidence  of  superior 
influence  of  the  sire  in  the  transmission  of  racial  fecundity. 


CHAPTER  XIX 


LINKAGE 


In  ordinary  Mendelian  inheritance,  if  two  characters,  A  and 
B,  enter  a  cross  in  the  same  gamete  (either  egg  or  sperm),  it 
will  be  wholly  a  matter  of  chance  whether  they  continue 
together  or  are  found  apart  in  the  following  generation.  If 
in  the  formation  of  gametes  by  the  cross-bred,  A  and  B 
separate  from  each  other  and  pass  into  different  gametes,  it 
is  evident  that  one  of  them  has  crossed-over  from  the  gametic 
group  in  which  both  originally  lay  to  enter  the  alternative 
group.  This  event  may  be  called  simply  a  crossover.  Cross- 
overs and  non-crossovers  will  be  equally  numerous  (50 
per  cent  each)  where  no  linkage  occurs.  Also,  if  A  and  B 
enter  a  cross  in  different  gametes,  one  in  the  egg,  the  other 
in  the  sperm,  it  will  in  ordinary  Mendelian  inheritance  be  a 
matter  of  chance  whether  they  emerge  from  the  cross  to- 
gether or  apart.  If  together,  it  is  evident  that  a  crossover 
has  occurred;  if  apart,  a  non-crossover,  that  is  a  persistence 
of  their  previous  relations.  Again,  crossovers  and  non- 
crossovers  will  be  equally  numerous  (50  per  cent  each)  if 
no  linkage  occurs. 

Linkage  may  be  defined  as  the  tendency  sometimes  shown 
by  genes  to  maintain  in  hereditary  transmission  their  previ- 
ous relations  to  each  other.  Thus  if  two  linked  genes,  A  and 
B,  enter  a  cross  together  in  the  same  gamete,  they  will 
oftener  than  not  be  found  together  in  the  gametes  formed 
by  the  cross-bred  individual.  Crossovers  in  that  case  will 
be  less  than  50  per  cent,  and  non-crossovers  more.  And  if 
the  same  two  genes  enter  the  cross  separately,  one  in  the  egg, 
the  other  in  the  sperm,  then  oftener  than  not  they  will  be 
found  apart,  in  different  gametes  formed  by  the  cross-bred 
individual.    Again  crossovers  will  be  less  than  50  per  cent. 

The  number  of  genes  in  a  linkage  group  varies  in  known 

167 


168  GENETICS  AND  EUGENICS 

cases  from  2  to  50  or  more.  However  many  genes  there  are 
in  a  linkage  group,  each  gene  shows  linkage  with  every  other 
gene  belonging  to  the  same  group,  but  the  apparent  strength 
of  the  linkage  varies  greatly.  Under  uniform  environmental 
conditions,  A  and  B  show  a  fairly  constant  hnkage  with  each 
other,  A  and  C  show  a  different  and  likewise  fairly  constant 
linkage  strength,  and  so  on  through  the  entire  group.  This 
leads  to  the  conclusion  that  the  genes  of  a  linkage  system 
are  bound  together,  gene  with  gene,  with  bonds  of  definite 
strength  in  each  case.  In  order  to  visualize  the  matter  and 
get  a  more  objective  view  of  linkage  relations,  Morgan  and 
his  associates  have  developed  the  chromosome__theory_of 
linkage,.  Its  essential  parts  are: 


1)  Genes  which  show  linkage  with  each  other  are  located 
in  the  same  pair  of  chromosomes.  It  is  the  substance  of  the 
chromosome  which  binds  the  genes  to  each  other  and  causes 
A  to  be  inherited  when  B  is. 

(2)  Genes  close  together  in  the  same  chromosome  show 
strong  linkage,  genes  farther  apa^t  show  less  linkage. 

(3)  Homologous    chromosomes,)  those    containing    corre- 
^sponding  sets  of  genes,  one  set  derived  from  the  father,  one 

from  the  mother,  lie  side  by  side  (in  synapsis)  previous  to 
the  formation  of  gametes.  At  this  time  breaks  are  likely  to 
occur  in  the  chromosomes  and  parts  of  one  are  likely  to  re- 
place corresponding  parts  of  the  other. 

(4)  Such  replacement  is  called  crossing-over. 

(5)  Breaks  are  commoner  in  long  chromosomes  than  in 
short  ones,  and  between  distant  points  than  between  near 
points  on  the  same  chromosome. 

(6)  The  genes  occur  in  a  chromosome,  like  beads  on  a 
string,  in  a  single  row  and  in  definite  order. 

The  supposed  order  of  the  genes  in  the  four  linkage  groups 
of  Drosophila  and  their  relative  distances  apart  are  shown 
in  Fig.  119.  In  these  diagrams,  or"maps,"when  the  probable 
order  of  the  genes  in  a  system  has  once  been  determined,  the 
supposed  end  gene  of  the  system  is  placed  at  position  0  and 
the  gene  next  to  it  is  placed  at  a  distance  (in  centimeters  or 


LINKAGE 


169 


other  units)  corresponding  to  the  average  cross-over  percent- 
age between  the  two,  this  process  being  repeated  from  gene 
to  gene  until  the  whole  chain  is  plotted.  The  "map"  is  thus 
based  on  a  summation  of  the  distances  (measured  in  cross- 
over percentages)  from  gene  to  gene.  But  if  we  compare  the 
"map  distances"  between  genes  not  adjacent  to  each  other 
in  the  chain  with  the  observed  cross-over  percentages  be- 
tween the  same  genes,  we  find  that  the  map  distance  is  regu- 
larly greater  than  the  cross-over  percentage,  except  for  very 
short  distances   (5  or  less) .    Thus  if  three  genes  occur  in  the 


I 


IV 


M 


a 


IV 


M 


Fig.  121a.  B  and  C  illustrate  Morgan's  idea  of 
the  linear  arrangement  of  the  genes  in  the  chromo- 
somes. A  and  D  show  how  the  composition  of 
chromosomes  is  supposed  to  change  as  a  result  of 
a  crossover.  Fig.  1'22.  A  pair  of  homologous 
chromosomes  a,  before;  6,  during;  c,  after  a  double 
crossover.    (After  Morgan.) 


order  A,  B,  C,  it  is  usually  found  that  AB  +  BC  is  greater 
than  AC.  In  other  words,  the  cross-over  percentage  be- 
tween A  and  B  plus  the  cross-over  percentage  between  B 
and  C  is  commonly  greater  than  the  cross-over  percentage 
between  A  and  C,  and  the  discrepancy  increases  with  the 
magnitude  of  the  values  involved.  This  fact  has  been 
accounted  for  in  two  different  ways.  First,  it  may  be  sup- 
posed that  the  arrangement  of  the  genes  is  really  not  linear, 
that  B  lies  out  of  line  with  A  and  C,  so  that  AC  will  be  less 
than  the  sum  of  AB  and  BC,  and  that  the  more  distant 
genes  are  no  farther  apart  than  indicated  by  the  cross-over 
percentages  between  them.     This  explanation  has  met  with 


170  GENETICS  AND  EUGENICS 

more  difficulties  than  it  has  cleared  away.  The  second  expla- 
nation  is   that  the    map-distances    indicate    proportionate 
numbers  of  breaks  in  the  linkage  chain  between  points,  not 
proportionate  numbers  of  changes  of  relation  between  genes 
at  particular  points.   Thus,  suppose  genes  AB  C  D  E  of  a  link- 
age system  meet  their  allelomorphs,  a  b  c  d  e,  in  a  cross  and 
gametes  are  later  formed  by  the  cross-bred  as  follows,  (1) 
A  B  c  d  e,  (2)  A  B  c  d  E,  and  (3)  A  b  c  D  e.   Assuming  that 
the  arrangement  is  linear,  we  must  suppose  that  one  break  in 
the  linkage  chain  has  occurred  in  (1),  two  breaks  in  (2),  and 
three  breaks  in  (3).    But  if  we  did  not  have  genes  BCD 
under  observation,  and  merely  noted  the  relation  of  A  to  E, 
we  should  infer  that  in  case  (1)  and  in  case  (3)  a  single  cross- 
over had  occurred,  but  that  in  case  (2)  no  crossover  had  oc- 
curred.   We  should  on  that  basis  underestimate  the  amount 
of  breaking  in  the  linkage  chain.    Accordingly  the  construc- 
tion of  maps  on  the  basis  of  short  distances  summated  is 
justifiable,  provided  the  arrangement  is  linear,  as  it  seems 
to  be.   But  it  must  be  borne  in  mind  that  the  map  distances 
do  not  correspond  with   cross-over  percentages    (although 
they  are  based  on  them)  except  in  the  case  of  very  short 
distances.    Map  distances  often  exceed  50,  but  cross-over 
percentages  can  not  do  so,  as  already  pointed  out.    To  get 
a  distinctive  name  for  the  map  units,  Haldane  has  called 
them  units  of  Morgan  or  simply  ** morgans."    Haldane  has 
computed  a  formula  for  converting  cross-over  percentages 
into  ** morgans"  and  vice  versa.    He  finds  that  the  two  cor- 
respond only  for  very  low  values   (5  or  less)  and  diverge 
more  and  more  as  the  observed  cross-over  percentages  ap- 
proach 50.    Haldane's  formula  may  be  stated  thus.    If  three 
genes.  A,  B,  and  C,  occur  in  a  common  linkage  group,  and 
the  cross-over  percentages  are  known  between  A  and  B  and 
between  B  and  C,  we  may  predict  with  a  probable  error  of 
not  over  two  per  cent,  what  cross-over  percentage  will  be 
found  to  occur  between  A  and  C.    Calling  the  cross-over 
percentage  between  A  and  B,  m,  and  that  between  B  and  C, 
n,  the  cross-over  percentage  between  A  and  C  will  lie  be- 


LINKAGE  171 

tween  (m  +  n)  and  (m  +  n  -  2mn) .  It  will  approach  the 
former  for  amounts  of  5  or  less,  and  the  latter  for  amounts 
of  45  or  over.  In  a  useful  table  Haldane  shows  the  calculated 
map  distances  (morgans)  for  all  cross-over  percentages  be- 
tween 5  and  50.  This  table  is  based  on  the  relations  of  the 
genes  observed  in  the  sex-linked  group  of  Drosophila,  but  it 
applies  equally  well  to  the  second  linkage  group  of  Droso- 
phila and  to  a  group  of  three  genes  in  the  plant,  Primula. 
Provisionally  it  may  be  considered  to  be  applicable  generally 
to  linkage  systems  in  animals  and  in  plants. 


TABLE  24 

A   Table   for   Converting 

Cross-Over   Percentages 

i    INTO 

Map  Distances 

("Morgans") 

AND   V 

ICE  Versa. 

After  Haldane 

Cross-over  percentage     0.0 

5.0 

8.0 

10.0 

11.0 

12.0 

13.0 

Map  distance                   0.0 

5.1 

8.2 

10.3 

11.4 

12.5 

13.6 

14.0           15.0         16.0 

17.0 

18.0 

19.0 

20.0 

21.0 

22.0 

14.7           15.9         17.0 

18.1 

19.3 

20.5 

21.7 

22.9 

24.1 

23.0          24.0        25.0 

26.0 

27.0 

28.0 

29.0 

30.0 

37.0 

25.3          26.6        27.9 

29.2 

30.5 

31.9 

33.3 

34.7 

36.2 

32.0          33.0        34.0 

35.0 

36.0 

37.0 

38.0 

39.0 

40.0 

37.7          39.3        40.9 

42.6 

44.3 

46.1 

48.0 

50.0 

52.2 

41.0          42.0        43.0 

44.0 

45.0 

46.0 

47.0 

48.0  ■ 

49.0 

54.4           56.8         59.6 

62.6 

66.0 

70.1 

75.1 

81.9 

93.0 

49.5          49.7        49.8 

49.9 

50.0 

99.2        109.4       117.7 

128.1 

oc 

As  an  example  of  how  the  table  may  be  used  in  predicting 
undetermined  linkage  values,  suppose  that  A  is  linked  with 
B,  and  B  with  C  and  that  between  A  and  B  there  are  10  pvr 
cent  of  crossovers,  and  between  B  and  C,  15  per  cent  of 
crossovers.  What  will  be  the  cross-over  percentage  between 
A  and  C.^  Converting  the  observed  cross-over  percentages 
into  map  distances  with  the  aid  of  the  table,  we  find  the 
distance  AB  to  be  10.3  and  the  distance  BC  to  be  15.9.  On 
the  linear  theory  the  distance  AC  will  equal  either  the  sum 
or  the  difference  of  AB  and  BC,  that  is  will  be  either  '■26.''Z  or 
5.4.  Converting  these  distances  into  cross-over  percentiiges 
by  interpolation  in  the  table,  we  find  that  the  cross-over 


172  GENETICS  AND  EUGENICS 

percentage  between  A  and  C  should  be  either  23.7  or  5.1, 
according  as  the  hnear  arrangement  is  ABC  or  ACB. 

Measurement  of  linkage.  It  will  be  observed  that  as  the 
strength  of  linkage  increases,  the  cross-over  percentage  de- 
creases. With  a  cross-over  percentage  of  50,  there  is  no 
linkage.  With  a  cross-over  percentage  of  0,  the  linkage  is 
complete,  two  characters  so  related  behaving  as  allelomorphs. 
Accordingly  we  depend  upon  the  observed  cross-over  per- 
centage both  for  the  detection  of  linkage  and  for  the  measure- 
ment of  its  strength.  But  unfortunately  the  linkage  strength 
varies  inversely  as  the  cross-over  percentage.  This  makes 
the  cross-over  percentage  directly  considered,  a  rather  poor 
measure  of  linkage  strength.  It  is  really  the  amount  by 
which  the  cross-over  percentage  falls  below  50  that  measures 
directly  the  strength  of  linkage.  Thus  with  cross-over  per- 
centages of  50,  40,  30,  20,  10,  and  0,  we  should  have  linkage 
strengths  of  0,  10,  20,  30,  40,  and  50.  We  should  then  have 
a  standard  for  measuring  linkage  strength  directly,  on  a 
scale  of  50.  But  as  we  are  more  accustomed  to  grading  on  a 
scale  of  100,  it  seems  preferable  to  double  the  values  indicated 
above.  We  then  have  grades  of  linkage  strength  on  a  scale 
of  100,  as  follows: 


Cross-over  Percentage 
50 

Linkage  Strength 
0 

40 

20 

30 

40 

20 

60 

10 

80 

0 

100 

Accordingly,  to  estimate  the  strength  of  linkage  in  a  particu- 
lar case,  we  multiply  by  2  the  difference  between  the  ob- 
served cross-over  percentage  and  50. 

But  suppose  the  observed  cross-over  percentage  were 
greater  than  50,  what  then.^  Such  an  occurrence  would  not 
indicate  linkage,  a  tendency  of  characters  to  remain  grouped 
as  they  were,  but  an  opposite  tendency,  to  assume  new  group- 
ings. No  such  tendency  has  been  observed.  If  it  should  be, 
it  would  need  a  different  name  and  method  of  measurement. 


LINKAGE  173 

We  may  now  consider  some  further  examples  of  linkage. 
In  the  plant,  Primula  sinensis,  Gregory  observed  the  oc- 
currence of  linkage  in  a  group  of  five  characters,  viz. 

Dominant  Recessive 

1.  Short  style  vs.  long  style  (1). 

2.  Magenta  corolla  vs.  red  corolla  (r). 

3.  Tinged  corolla     vs.  full-colored  corolla. 

4.  Green  stigma       vs.  red  stigma  (s). 

5.  Pale  stem  vs.  full  red  stem. 

Altenburg  later  determined  the  strength  of  the  linkage  ex- 
isting between  three  of  these  five  pairs  of  characters,  viz.,  1, 
2,  and  4  of  the  above  list.  His  results  may  be  expressed  in 
a  linkage  map  as  follows: 


0  34.0        45.6 

The  cross-over  percentage  between  1  and  r  was  found  to  be 
34.02,  between  r  and  s,  11.62.  The  sum  of  these  two,  45.64, 
is  the  total  (uncorrected)  map  distance.  The  observed  cross 
over  percentage  between  1  and  s  was  40.6,  which  falls  short 
of  the  map  distance  by  almost  exactly  the  amount  indicated 
by  Haldane's  table. 

In  the  sweet  pea  the  earliest  discovered  examples  of  link- 
age are  found.  Here  are  known  two  linkage  groups  con- 
taining each  three  pairs  of  characters  as  follows; 

Dominant  Recessive 

1.    Blue  vs.  red  flower  color. 

Group  I.       2.    Long  vs.  round  pollen. 

3.    Erect  vs.  hooded  standard. 

1.    Dark      vs.  light  leaf-axil. 
Group  II.     2.    Fertile    vs.  sterile  anthers. 
3.    Normal  vs.  cretin  flowers. 

Results  described  by  Bateson  and  by  Punnett  indicate  that 
in  Group  I  the  map  relations  of  the  three  genes  are: 

E-B L 

0       .78  12.5 

The  group  is  a  compact  one,  with  E  and  B  very  closely 
linked,  cross-over  percentage  less  than  one,  with  B  and  L 


174  GENETICS  AND  EUGENICS 

showing  between  11  and  12  per  cent  crossovers,  and  with 
E  and  L  showing  about  12.5  per  cent  of  crossovers. 

In  Group  II,  the  cross-over  percentage  between  D  and  F 
is  about  6.2,  between  F  and  N  about  25.0.  Until  the  cross- 
over percentage  between  D  and  N  has  been  experimentally 
determined,  it  cannot  be  stated  whether  the  **map"  order 
is  F  D  N  or  F  N  D.  In  the  former  case,  the  total  map 
distance  will  be  25,  or  about  double  the  length  of  Group  I; 
in  the  latter  case,  it  will  be  still  longer,  or  about  31.2. 

In  garden  peas  two  independent  pairs  of  linked  characters 
are  known  and  two  more  are  suspected  (Wliite).  In  one  of 
the  established  cases  close  linkage  is  found  between  round 
starchy  seeds  and  tendrils  on  the  leaves,  with  about  1.5  per 
cent  of  crossing-over.  In  the  other  case  a  gene  for  late  flow- 
ering is  linked  with  red  flower  color  with  an  estimated  cross- 
over percentage  of  between  12  and  16. 

In  the  snapdragon.  Antirrhinum,  two  factors  for  flower 
color  were  found  by  Baur  to  be  linked,  with  about  20  per 
cent  of  crossovers  occurring. 

In  maize  three  linkage  groups  are  known,  one  of  four 
factors  and  two  of  two  factors  each.  Group  1  includes  a 
factor  for  waxy  endosperm  and  the  factor  C  for  aleurone 
color.  These  show  a  cross-over  percentage  of  26.7.  Group  2 
includes  fowr  linked  factors,  aleurone  factor  R,  chlorophyl 
factor  G,  chlorophyl  factor  L,  and  aleurone  spotting  factor, 
S.  No  crossovers  have  been  observed  between  R  and  L 
which  behave  as  if  they  were  allelomorphs,  or  **  completely 
linked."  The  cross-over  percentage  between  L  and  G  has 
been  determined  as  23,  that  between  R  and  G  has  been  de- 
termined less  accurately  as  19,  and  that  between  R  and  S 
as  12.5.    The  order  of  the  genes  is  accordingly 

RL S G. 

Group  3  includes  the  two  characters,  starchy  endosperm 
and  tunicate  ("podded")  seeds.  The  cross-over  percentage 
in  this  case  is  8.3  (Jones  and  Gallastegui). 

In  the  cultivated  tomato  two  cases  of  linkage  have  been 
reported.    A  gene  for  "standard"  vine  habit  and  a  gene  for 


LINKAGE 


175 


TABLE  25 
Cases  of  Linkage  in  Plants  or  in  Anevlvls  other  than  Drosophila 


Species 

Group 

Linked  Characters 

Cross-over  ,  Linkage  i         .    »l     •. 
Percentage'  Strength         Authority 

Sweet 
pea 

1 

1 
1 

2 

2 

2 

Purple  flowers,  long  pollen 
Purple  flowers,  erect  standard 
Long  pollen,  erect  standard 
Dark  axil,  fertile  anthers 
Dark  axil,  normal  (not  cretin) 

flower 
Fertile    anthers,    normal    (not 

cretin)  flowers 

11  or  12 
0.78 
12.5 
6.2 

5 

* 
• 

25.0 

76-78 

98.4 

75 

87.6 

50 

Bateson 

and 

Punnett 

Primula 
sinensis 

Short  style,  magenta  corolla 
Short  style,  green  stigma 
Magenta  corolla,  green  stigma 
Tinged  corolla,  green  stigma 
Pale  stem,  green  stigma 

34.0 
40.6 
11.6 

• 
• 

32 

18.8 
76.8 

Altenburg 

(( 

<< 
Gregory 

Garden 
pea 

2 

Round  seeds,  tendrils  on  leaves 
Late  flowering,  colored  flowers 

1.5 
12-16 

97 
68-76 

Bateson  and 

Vilmorin 

Hoshino 

Antirrhi-        1 
num 

Red  flower  color,  "picturatum  " 
pattern 

20.0              60 

Baur 

Maize 

1 
2 
2 
2 
3 

Waxy  endosperm,  Aleurone  C 
Aleurone  R,  Chlorophyl  G 
Aleurone  R,  Chlorophyl  L 
Chlorophyl  G,  Chlorophyl  L 
Starchy  endosperm,  tunicate 
seed 

26.7 
19.0 

0.0 
23.0 

8.3 

46.6 
62? 
100 
54 

83.4 

Breggar 
Lindstrom 

it 

Jones 

Tomato          1 

2 

Vine  habit,  fruit  shape 
Green  foliage,  2-celled  fruit 

20.0     1         60     Jones 
0?            100.^ 

Beans        j     1 

Seed  pattern.  Vine  habit              '       0? 

100?     Surface 

Silkworm 

1 

Pattern  Q  of  larva,  yellow  silk 

26.1 

47.18 

Tanaka 

Apotettix 

1 
1 
1 
1 

1 
1 
1 
1 

Patterns  G  and  M 

M  «    K 

K    «    Y 

Y    "    R 

u         Y    "     T 

R    "    T 

"        M  ''    R 

«        Y    "     Z 

4  (in  9) 

1    " 

6    " 
10    " 
12    " 

0    " 
10    " 

92 
98 
88 
80 
76 
100 
80 
80 

Nabours 

u 
u 
u 
u 
u 
u 
u 

Pigeon 

1 

Sex-linked  factors  I  and  A 

40(in  d")          20  1  Cole  and 

Kelley 

Rat 

1 
1 
1 

Albinism,  red-eye                           '        1-0?             98?'  Castle  and 
Albinism,  pink-eye                        ;     21.0              58     Dunn 
Red-eye,  pink-eye                               18.3     j      63.4 

Mouse 

1 

Albinism,  pink-eye. 

14.3           71.4  1  Castle  and 

Dunn 

1 

176  GENETICS  AND  EUGENICS 

*' constricted"  fruit  shape  show  about  20  per  cent  of  crossing- 
over.  In  another  linkage  group,  no  crossovers  have  been 
observed  between  green  foHage  color  and  two-celled  fruit,  as 
opposed  to  yellow  foliage  color  and  many-celled  fruit,  in  a 
total  of  24  r2  plants.  It  seems  probable  that  the  linkage  in 
this  latter  case  is  close,  though  the  number  of  observations 
is  too  small  to  do  more  than  establish  a  probability. 

In  rats  a  group  of  three  linked  characters  has  been  found, 
albinism  (c),  red-e}^e  (r)  and  pink-eye  (p),  which  may  be 
mapped,  thus 

c_i._______     _____--      p 

0      1  20 

In  mice  albinism  (c)  and  pink-eye  (p)  are  linked,  as  they  are 
in  rats,  but  the  cross-over  percentage  is  less,  viz.,  14.3. 

In  the  silkworm,  linkage  occurs  between  a  factor,  Q,  which 
gives  to  the  larva  characteristic  pattern  markings,  and  a 
factor,  Y,  which  gives  to  the  blood  of  the  larva  and  the  silk 
of  the  cocoon  a  yellow  color.  Crossing-over  occurs  only  in 
males,  and  in  a  percentage  of  26.1  (in  a  large  series  of  back- 
crosses  of  Fi  hybrid  male  with  double  recessive  female,  pro- 
ducing 24,918  individuals).  In  Drosophila  crossing-over 
occurs  only  in  the  female  parent,  that  is  in  the  maturation 
of  the  eggs.  This  is  true  of  all  linkage  groups,  whether  they 
involve  sex-linkage  or  not.  In  the  grouse-locust,  Apotettix, 
a  linkage  group  of  seven  or  more  characters  has  been  dis- 
covered by  Nabours,  which  have  this  curious  feature,  that 
crossing-over  seems  to  occur  much  more  frequently  in  fe- 
males than  in  males.  In  all  other  known  cases  of  linkage, 
crossing-over  occurs  with  about  the  same  frequency  in  the 
gametes  formed  by  both  sexes.  This  accordingly  is  to  be 
regarded  as  the  normal  condition.  Failure  of  crossing-over 
to  occur  in  the  oogenesis  of  Drosophila  and  in  the  spermato- 
genesis of  the  silkworm  would  seem  to  imply  unusual 
cytological  conditions  in  those  cases. 


CHAPTER  XX 


THE  NATURE  OF  GENES 


When  a  pair  of  alternative  characters,  such  as  pigmentation 
and  albinism,  is  involved  in  a  cross,  we  assume  that  the 
gamete  which.  transmit5~T>ne  of  the  alternative  conditions 
differs  structurally  from  that  which  transmits  the  other  and 
that  this  structural  difference  is  the  cause  of  their  difl'erent 
powers  of  transmission.    By  the  study  of  linkage  relations 
we  find  that  the  structural  difference  is  confined  to  a  par- 
ticular linkage  group,  in  mice  and  rats  to  the  group  which 
also  includes  the  factor  for  pink-eyed  dilution.    If  we  adopt 
the  chromosome  hypothesis,  we  locate  the  structural  differ- 
ence in  a  particular  chromosome  and  suppose  that  it  exists 
in  a  definite  region  (or  locus)  of  that  chromosome.    Each 
structurally  different  state  of  a  locus  is  called  a  gene.    The 
color  gene  shows  the  alternative  forms  which  we  call  C  and 
c.    With  all  the  residual  heredity  unchanged,  C  will  cause  the 
development  of  full  pigmentation,  while  c  will  leave  the  skin 
unpigmented.    For  information  as  to  what  C  and  c  are,  we 
may  consult  the  biochemists,  who  have  devoted  considerable 
attention  to  the  chemical  processes  involved  in  pigment  for- 
mation.   Wright  (1917)  after  an  exliaustive  review  of  the 
chemical  evidence  concludes   (1)   '*that  melanin  (pigment) 
is  produced  by  the  oxidation  of  certain  products  of  protein 
metabolism  by  the  action  of  specific  enzymes,  (^2)  that  the 
reaction  takes  place  in  the  cytoplasm  of  cells  probably  by 
enzymes  secreted  by  the  nucleus,  (3)  that  various  chromo- 
gens  are  used,  the  particular  ones  oxidized  depending  on  the 
character  of  the  enzymes  present,  and  finally  that  hereditary 
differences  in  color  are  due  to  hereditary  differences  in  the 
enzyme  element  of  the  reaction."    The  final  conclusion  is  of 
particular  interest.   It  indicates  that  the  gene  C  is  concerned 
in  enzyme  production.   Wright  offers  a  provisional  hypolhe- 

177 


178  GENETICS  AND  EUGENICS 

sis  to  explain  variations  in  the  character  or  amount  of  pig- 
ment found  in  the  coats  of  mammals,  which  involves  two 
enzymes  acting  in  succession  in  the  oxidation  of  chromogens. 
Enzyme  I  performs  the  initial  action,  and  acting  by  itself 
produces  yellow  pigment  (known  also  as  red  or  cream,  ac- 
cording to  the  amount  of  pigment  formed) .   Enzyme  II  can- 
not act  on  chromogens  except  in  connection  with  Enzyme  I 
in  which  relation  it  carries  forward  the  oxidation  to  a  brown 
or  black  stage.    Without  the  presence  of  Enzyme  I,  no  pig- 
ment at  all  will  be  produced,  that  is  the  albino  state  will 
result,  even  though  Enzyme  II  is  present.  According  to  this 
hypothesis  the  gene  C  is  concerned  in  the  production  of 
Enzyme  I.  But  we  are  acquainted  with  several  allelomorphic 
forms  of  this  gene,  which  in  guinea-pigs  are  effective  re- 
spectively in  full  pigmentation,  dilute  pigmentation,  red- 
eyed  dilution,  and  Himalayan  albinism.    We  must  suppose 
that  in  this  series  of  mutations.  Enzyme  I  is  produced  more 
and  more  feebly,  until  in  complete  albinism  (as  seen  in  rab- 
bits, rats  and  mice)  no  effective  production  of  Enzyme  I 
occurs.    On  the  chromosome  theory  we  must  accordingly 
suppose  that  the  production  of  Enzyme  I  depends  upon  a 
structure  of  some  sort  (gene  C,  c,  etc.)  having  a  definite 
position   (locus)   in  a  particular  chromosome.    At  definite 
positions  in  this  same  chromosome,  we  must,  on  this  theory, 
locate  one  or  more  genes  which  influence  the  production  of 
Enzyme  II  in  the  rat  and  in  the  mouse.    In  both  the  rat  and 
the  mouse,  a  gene  for  pink-eye  (or  its  allelomorph,  dark  eye) 
is  linked  with  the  color  gene.    This  gene  (in  the  form  pink- 
eye) diminishes  greatly  the  amount  of  black  pigment  pro- 
duced in  eye  and  coat,  but  does  not  diminish  at  all  the 
amount  of  yellow  pigment  formed.    Hence  it  affects  the 
hypothetical  Enzyme  II  but  not  Enzyme  I.   In  the  rat,  an- 
other gene,  that  for  red-eyed  yellow,  linked  still  more  closely 
with  the  color  gene,  likewise  reduces  the  amount  of  black 
pigment  formed  in  the  coat  and  the  eye,  but  without  dimin- 
ishing at  all  the  production  of  yellow  pigment.  But  it  allows 
of  more  pigment  development  in  the  eye  than  does  the  gene 


THE  NATURE  OF  GENES  I79 

for  pink-eye,  and  this  is  indicated  in  the  name  **red-eye." 
That  the  genes  for  red-eye  and  pink-eye  are  different  in 
chemical  nature  is  shown  by  their  complementary  action. 
When  pink-eyed  and  red-eyed  rats  are  crossed,  black  pig- 
mented young  result. 

A  gene  which  in  mice  influences  the  action  of  Enzyme  II 
has  the  allelomorphic  forms  black  (B)  and  brown  (b).  It  is 
not  linked  with  the  color  gene  and  so  cannot  lie  in  the  same 
chromosome  with  it  (Little  and  Phillips,  Detlefsen).  Gene 
b  interrupts  the  action  of  Enzyme  II  when  the  pigment  has 
been  oxidized  to  a  chocolate  brown  color,  B  allows  the  oxi- 
dation to  continue  until  the  black  stage  is  reached. 

Another  gene  which  limits  the  action  of  Enzyme  11  is  the 
agouti  factor.  In  mice  it  is  not  linked  either  with  C  or  with 
B.  Hence  it  must  lie  in  a  third  chromosome.  It  restricts  the 
action  of  Enzyme  II  to  particular  parts  of  the  hair,  the  base 
and  tip  of  the  hair  in  most  body  regions,  and  on  the  belly 
the  base  alone,  or  it  may  exclude  the  action  of  Enzyme  II 
from  the  entire  hair  in  the  belly  region.  As  the  dominant 
allelomorph  of  the  agouti  factor,  the  gene  yellow  inhibits  the 
action  of  Enzyme  II  more  or  less  completely  throughout  the 
coat  of  mice. 

In  rabbits  and  guinea-pigs  a  gene  called  the  extension 
factor  (E,  e)  influences  the  production  or  action  of  Enzyme 
II.  As  E  it  permits  black  (or  brown)  pigment  to  be  pro- 
duced throughout  the  coat,  except  where  its  production  is 
interfered  with  by  the  agouti  factor.  As  e,  it  does  not  permit 
Enzyme  II  to  function  in  the  coat,  but  only  in  the  eyes  and 
skin.  Consequently  the  coat  is  yellow  through  the  unas- 
sisted action  of  Enzyme  I.  A  third  allelomorph,  e',  in  guinea- 
pigs  allows  Enzyme  11  to  act  in  part  of  the  coat  only,  thus 
producing  a  yellow-and-black  spotted  coat.  The  extension 
factor  is  apparently  not  linked  with  any  of  the  other  factors 
for  color  production,  and  so  must  be  located  in  a  fourth 
chromosome. 

How  many  other  genes  there  are  which  influence  the  action 
of  Enzyme  II,  we  do  not  know,  nor  do  we  know  wluit  their 


180  GENETICS  AND  EUGENICS 

nature  is,  but  it  would  seem  improbable  that  any  one  of 
them  is  itself  Enzyme  II,  but  only  that  it  is  in  some  way  con- 
cerned in  the  production  of  Enzyme  II,  either  locally  or 
generally. 

As  regards  Enzyme  I,  which  is  produced  in  several  grades 
(qualitative  or  quantitative)  through  mutations  in  the  color 
gene  resulting  in  multiple  allelomorphs,  we  know  that  its 
action  may  be  localized  by  other  independently  inherited 
genes,  those  not  in  the  same  linkage-group  or  chromosome. 
Such  are  the  factors  for  white  spotting  which  in  no  case  have 
been  shown  to  be  linked  with  albinism.  Some  factors  of  this 
sort  seem  to  interfere  with  the  production  of  Enzyme  I  in 
particular  parts  of  the  body,  others  allow  Enzyme  I  to  be 
produced  but  inhibit  its  action  in  particular  body  regions. 
Again  we  have  no  present  knowledge  as  to  what  the  nature 
of  these  modifying  genes  is.  In  Drosophila  there  occur  in  a 
single  linkage  system  (chromosome),  genes  affecting  various 
parts  of  the  body  and  affecting  them  in  various  ways.  Thus 
in  the  sex-linked  group  of  genes  are  found  those  which  in- 
fluence the  shape  of  the  eye,  the  color  of  the  eye,  the  length 
of  the  wings,  the  shape  of  the  wings,  the  venation  of  the 
wings,  the  form  of  the  legs,  the  color  of  the  body,  the  shape 
of  the  bristles  on  head  and  thorax,  the  form  of  the  abdomen, 
and  many  others  less  easy  of  description. 

Again,  in  the  '* second-chromosome"  linkage  group  of 
Drosophila,  are  found  other  genes  which  also  affect  practi- 
cally all  regions  of  the  body,  as  for  example,  shape,  size,  and 
venation  of  wings,  length  of  legs,  color  and  structure  of  the 
compound  eyes,  patterns  of  thorax,  shape  of  abdomen,  and 
general  body-color.  No  linkage  system  specializes  in  genes 
of  any  particular  sort,  or  affecting  any  particular  region  of 
the  body.  Often  a  single  gene  is  known  to  affect  various 
parts  of  the  body.  Thus  the  gene,  **dachs,"  affects  both  the 
length  of  the  legs  and  the  venation  of  the  wing. 

If  any  part  of  any  chromosome  of  an  egg  of  Drosophila 
were  removed  or  changed  in  composition,  it  seems  probable 
that  some  departure  from  normality  would  follow  in  the  fly 


THE  NATURE  OF  GENES  181 

which  developed  from  the  egg.  In  that  case  the  chromosome 
change  might  be  regarded  as  a  gene  responsible  for  the  ob- 
served departure  from  normality.  As  such  it  would  behave 
in  crosses  with  normal  individuals.  If  this  is  true,  it  seems 
probable  that  the  entire  chromatin,  or  at  least  so  much  of 
it  as  is  concerned  in  determining  the  activities  of  the  cell, 
may  be  regarded  as  composed  of  genes.  A  gene  will  be  the 
smallest  part  of  the  chromatin  capable  of  varying  by  itself. 
And  if  the  gamete  contains  any  structures  not  chromatin 
which  are  concerned  in  heredity,  that  is  which  are  repro- 
duced when  the  cell  divides,  these  too  will  constitute  genes. 
Further  investigation  alone  can  show  whether  or  not  genes 
are  found  exclusively  in  the  chromatin.  At  present  it  is 
assumed  that  such  is  the  case. 


CHAPTER  XXI 

ARE  UNIT-CHARACTERS  (GENES)   CONSTANT  OR 

VARLABLE? 

In  some  of  the  preceding  chapters  we  have  considered  facts 
which  show  to  what  a  large  extent  the  varieties  of  animals  and 
plants  formed  under  domestication  owe  their  origin  to  dis- 
continuous variations  or  sports,  which,  by  reason  of  their 
Mendelian  behavior  in  heredity,  may  be  combined  in  various 
ways  through  the  agency  of  hybridization.  It  is  a  question  of 
much  interest,  both  theoretical  and  practical,  whether  these 
sports  or  unit-character  variations,  are  entirely  stable  or 
whether  they  themselves  are  subject  to  variation.  For  if  a 
unit-character  is  not  variable,  we  can  only  vary  the  combi- 
nations into  which  it  enters,  the  character  itself  being  un- 
affected. But  if  a  unit-character  is  variable,  it  is  important 
to  know  whether  its  variation  is  continuous  or  discontinuous. 
For  if  it  varies  by  distinct  steps  only,  that  is  discontinuously, 
it  would  be  a  waste  of  time  to  try  by  selection  to  establish 
any  other  conditions  than  those  which  arise  spontaneously, 
by  "  mutation  "  as  De  Vries  would  say. 

The  mutation  idea  has  greatly  weakened  the  faith  of  biolo- 
gists in  selection.  Darwin  had  great  confidence  in  the  power 
of  selection  gradually  to  modify  the  characteristics  of  races. 
Practical  breeders  of  animals  and  plants  have  always  worked 
by  this  means,  and  Darwin  based  his  views  concerning  the 
eflScacy  of  selection  largely  on  the  results  of  their  experience. 
But  breeders  do  not  confine  their  attention  to  the  propaga- 
tion of  variations  which  they  have  seen  arise  spontaneously. 
They  often  form  ideals  of  uncreated  varieties  and  then  work 
zealously  for  the  production  of  these.  Some  of  these  ideals 
may  be  unattainable,  but  too  many  of  them  have  been  real- 
ized to  make  us  think  that  all  work  of  this  sort  is  fruitless. 
Today  animal  breeders  hold  among  their  unrealized  ideals, 
a  tri-color  variety  of  mouse;  a  blue  variety  of  fowl  which  will 

182 


Fig.  123.  English  rabbits  showing  a  dominant  form  of  white  spotting  which  fluctuates  both  somatically  and  gene- 
tically. The  first  five  figures  were  employed  as  grades  1-5  in  classifying  observed  fluctuations.  The  third  figure 
(middle  row,  left)  is  close  to  the  fancier's  ideal  English  marking.  The  two  rabbits  shown  at  the  top  and  the  one  ut 
the  right,  bottom,  were  homozygous  for  the  English  pattern;  the  other  three  were  heterozygous.  English  pattern 
is  allelomorphic  to  Dutch,  Fig.  138. 


ARE  UNIT-CHARACTERS  CONSTANT  ?  183 

breed  true,  as  blue  pigeons  do;  a  race  of  barred  Pl^-mouth 
Rock  fowls  of  the  same  color  in  both  sexes.  These  ideals  the 
student  of  genetics  says  are  unattainable  and  he  can  gi\'e  good 
reasons  for  so  regarding  them.  Nevertheless  breeders  will 
doubtless  continue  to  try  for  them  and  it  is  hardly  safe  to  say 
that  success  is  impossible.  Most  advances  in  practical  affairs 
are  made  by  those  who  have  the  courage  to  attempt  what 
others  with  good  reason  think  unattainable.  When  such 
attempts  have  succeeded,  the  world  simply  revises  its  classifi- 
cation of  things  attainable  and  unattainable,  and  makes  a 
fresh  start. 

Many  students  of  genetics  at  present  regard  unit-characters 
as  unchangeable.  They  consider  them  as  impossible  of  modi- 
fication as  are  the  atoms.  To  recall  Bateson's  comparison, 
the  carbon  and  oxygen  of  carbon  monoxide,  CO,  are  each 
unchangeable.  Adding  another  atom  of  oxygen  does  not 
alter  them,  though  it  changes  radically  the  compound  fonned 
which  becomes  carbon  dioxide,  CO2,  possessed  of  very  differ- 
ent properties.  But  the  carbon  and  the  oxygen  are  still  there, 
unaltered  and  recoverable.  This  question  is  one  of  great 
practical  importance,  —  are  unit-characters  as  constant  as 
atoms,  so  that  we  can  merely  recombine  them,  or  are  they 
different  in  nature  from  atoms  so  that  we  can  modify  as  well 
as  recombine  them.  Much  careful  work  has  been  devoted  to 
the  solution  of  this  question.  It  was  at  first  assumed  from 
chemical  analogy  that  characters  which  behave  as  units  in 
heredity  must,  like  C  and  O  in  the  case  of  carbon  dioxide, 
emerge  from  combinations  unmodified.  But  presently  case 
after  case  came  to  light  in  which  this  was  not  true.  Albin- 
ism emerged  from  crosses  tainted  with  color;  clear  yellows 
emerged  from  crosses  intensified  to  red,  or  diluted  to  cream, 
or  sooty  with  minute  quantities  of  black;  patterns  such  as 
are  seen  in  Dutch  or  in  English  rabbits,  or  in  hooded  rats, 
emerged  considerably  altered  in  appearance.  Facts  such  as 
these  were  interpreted  in  two  different  ways.  It  was  as- 
sumed by  some  that  the  actual  unit-character,  factor,  or 
gene  involved  was  subject  to  quantitative  and  possibly  to 


184 


GENETICS  AND  EUGENICS 


qualitative  change.  By  others  it  was  assumed  that  the  ob- 
served character  changes  were  not  due  to  changes  in  single 
genes  but  to  the  supplemental  or  modifying  action  of  the 
other  genes.  For  example,  the  hooded  pattern  of  rats  (Figs. 
124  and  125)  clearly  behaves  as  a  simple  unit-character  alle- 
lomorphic  to  Irish  pattern  or  to  self  in  crosses.  But  the 
hooded  pattern  as  seen  either  in  pure-bred  or  in  cross-bred 
litters  of  young  (Fig.  124)  varies  slightly,  and  such  variations 
have  a  genetic  basis  since  by  selecting  either  the  whitest  or 


Fig.  124.  Inheritance  of  a  recessive  pattern  of  white  spotting  seen  in  "  hooded  "  rats.  The  parents 
(at  the  left)  are  a  homozygous  hooded  mother  and  a  heterozygous  "  Irish  "  father  (black  with  white 
belly).  An  entire  litter  of  their  young  is  shown  at  the  right.  Four  are  homozygous  hooded  like  the 
mother,  five  are  heterozygotes  like  the  father.  Note  fluctuation  in  both  classes.  Such  fluctuations 
are  found  to  be  in  part  heritable. 

the  blackest  individuals,  one  can  either  whiten  or  blacken 
the  average  racial  condition.  (See  Tables  24a  and  25a.) 
Races  corresponding  with  the  extremes  of  the  series  shown  in 
Fig.  125  were  thus  produced.  The  question  now  arose 
whether  the  observed  changes  had  occurred  as  a  result  of 
change  in  the  single  unit-character  or  gene  clearly  concerned 
in  the  case,  or  whether  this  was  due  to  other  agencies.  To 
test  the  matter  the  selected  races,  now  modified  genetically 
in  opposite  directions,  were  crossed  repeatedly  with  a  non- 
hooded  (wild)  race.  The  recessive  hooded  character  dis- 
appeared in  Fi  but  was  recovered  again  in  F2  in  the  expected 
25  per  cent  of  this  generation.  Compare  Fig.  56.  These 
extracted  hooded  individuals,  following  each  cross,  were  less 
divergent  than  their  hooded  grandparents  from  the  ordinary 
hooded  pattern.    After  three  successive  crosses  (six  genera- 


ARE  UNIT-CHARACTERS  (GENES)  CONSTANT?    185 

tions)  the  whitest  individuals  extracted  from  the  dark  hooded 
race  were  no  darker  than  the  darkest  individuals  extracted 
from  the  white  hooded  race.  In  other  words  repeated  cross- 
ing with  the  non-hooded  (wild)  race  had  caused  the  changes 
in  the  hooded  character,  which  had  been  secured  by  selection, 
altogether  to  disappear.  This  result  shows  conclusively  that 
the  changes  in  question  had  not  occurred  in  the  gene  for  the 
hooded  pattern,  but  in  the  residual  heredity.  Other  cases  of 
apparent  gradual  change  in  unit-characters  under  the  action 
of  selection  may  be  explained  in  a  similar  way.  Accordingly 
we  are  led  to  conclude  that  unit-characters  or  genes  are  re- 


FiG.  125.    A  series  of  grades  for  classifying  the  plus  and  minus  variations  of  the  white 

spotting  pattern  of  hooded  rats. 


markably  constant  and  that  when  they  seem  to  change  as  a 
result  of  hybridization  or  of  selection  unattended  by  hybridi- 
zation, the  changes  are  rather  in  the  total  complex  of  factors 
concerned  in  heredity  than  in  single  genes. 

Nevertheless  changes  do  sometimes  occur  in  single  genes. 
Such,  we  assume,  are  the  several  unit-character  variations 
described  in  previous  chapters,  which  form  the  basis  of  the 
varieties  of  domestic  animals  and  cultivated  plants.  These 
occur  singly  and  sporadically  as  changes  each  in  a  particular 
locus  or  part  of  a  system  of  genes.  By  hybridization  these 
isolated  changes  are  later  combined  in  any  desired  fashion. 
Change  in  a  genetic  locus,  that  is  the  appearance  of  a  new 
gene,  is  in  the  terminology  of  Morgan  called  a  mutation  but 
this  use  of  the  term  differs  fundamentally  from  that  of  De 
Vries.  There  is  no  known  means  by  which  a  mutation,  in  this 


186 


GENETICS  AND  EUGENICS 


sense,  can  be  brought  about.  Genes  are  discovered,  not 
made  in  laboratories,  and  may  be  manipulated  by  hybridiza- 
tion but  not  changed.  The  suddenness  of  their  coming  and 
their  stability  are  implied  in  the  term  mutation. 

Sometimes  a  single  genetic  locus  may  undergo  several  dif- 
ferent mutations,  but  these,  so  far  as  we  know,  occur  in- 

TABLE  24a 

Results  of  the  Plus  Selection  of  Hooded  Rats  Continued  through 

Twenty  Successive  Generations 


Generation 

Mean  Grade 
of  Parents 

Mean  Grade 
of  Offspring 

Lowest 

Grade 

of  Offspring 

Highest 

Grade 

of  Offspring 

Standard 

Deviation  of 

Offspring 

Number  of 
Offspring 

1 

2.51 

2.05 

+  1.00 

+3.00 

.54 

150 

2 

2.52 

1.92 

-1.00 

+3.75 

.73 

471 

3 

2.73 

2.51 

+   .75 

+4.00 

.53 

341 

4 

3.09 

2.73 

+  .75 

+3.75 

.47 

444 

5 

3.33 

2.90 

+  .75 

+4.25 

.50 

610 

6 

3.52 

3.11 

+  1.50 

+4.50 

.49 

861 

7 

3.56 

3.20 

+  1.50 

+4.75 

.55 

1,077 

8 

3.75 

3.48 

+  1.75 

+4.50 

.44 

1,408 

9 

3.78 

3.54 

+  1.75 

+4.50 

.35 

1,322 

10 

3.88 

3.73 

+2.25 

+5.00 

.36 

776 

11 

3.98 

3.78 

+2.75 

+5.00 

.29 

697 

12 

4.10 

3.92 

+2.25 

+5.25 

.31 

682 

13 

4.13 

3.94 

+2.75 

+5.25 

.34 

529 

14 

4.14 

4.01 

+2.75 

+5.50 

.34 

1,359 

15 

4.38 

4.07 

+2.50 

+5.50 

.29 

3,690 

16 

4.45 

4.13 

+3.25 

+5.87 

.29 

1,690 

17 

4.81 

4.48 

+3.75 

+5.75 

•    • 

351 

18 

4.80 

4.46 

+3.50 

+5.50 

•    • 

420 

19 

4.66 

4.49 

+3.50 

+5.50 

•    • 

280 

20 

4.66 

4.61 

+3.75 

+5.75 

•    • 

92 

Total 

17,250 

dependently,  at  different  times  or  places,  and  cannot  be 
combined  for  the  reason  that  they  behave  as  allelomorphs 
in  crosses.  For  this  reason  not  more  than  two  of  them  can  be 
brought  into  the  same  zygote,  nor  more  than  one  into  the 
same  gamete.    We  call  them  multiple  allelomorphs. 


I 


UNIT-CHARACTERS  AND  SELECTION  187 

A  good  example  of  multiple  allelomorphism  is  found  in 
the  several  mutations  which  the  color  factor  of  rodents  has 
undergone.  This  is  the  factor  which  in  its  best  known  muta- 
tion assumes  the  form  of  albinism.   In  guinea-pigs  four  alle- 


TABLE  25a 

Results  of  the  Minus  Selection'  of  Hooded  Rats  Continued  through 

Twenty-one  Successive  Generations 


Generation 

Mean  Grade 
of  Parents 

Mean  Grade 
of  Offspring 

Lowest 

Grade 

of  Offspring 

Highest 

Grade 

of  Offspring 

Standard 

Deviation  of 

Offspring 

Number  of 
Offspring 

1 

-1.46 

-1.00 

+   .25 

-2.00 

.51 

55 

2 

-1.41 

-1.07 

+   .50 

-2.00 

.49 

132 

3 

-1.56 

-1.18 

0. 

-2.00 

.48 

195 

4 

-1.69 

-1.28 

+   .50 

-2.25 

.46 

329 

5 

-1.73 

-1.41 

0. 

-2.50 

.50 

701 

6 

-1.86 

-1.56 

0. 

-2.50 

.44 

1,252 

7 

-2.01 

-1.73 

0. 

-2.75 

.35 

1,680 

8 

-2.05 

-1.80 

0. 

-2.75 

.28 

1,726 

9 

-2.11 

-1.92 

-    .50 

-2.75 

.28 

1,591 

10 

-2.18 

-2.01 

-1.00 

-3.25 

.24 

1,451 

11 

-2.30 

-2.15 

-1.00 

-3.50 

.35 

984 

12 

-2.44 

-2.23 

-1.00 

-3.50 

.37 

1,037 

13 

-2.48 

-2.39 

-1.75 

-3.50 

.34 

1,006 

14 

-2.64 

-2.48 

-1.00 

-3.50 

.30 

717 

15 

-2.65 

-2.54 

-1.75 

-3.50 

.29 

1,438 

16 

-2.79 

-2.63 

-1.00 

-4.00 

.27 

1.980 

17 

-2.86 

-2.70 

-1.75 

-4.25 

.28 

868 

18 

-3.09 

-2.84 

-2.25 

-4.00 

.    . 

330 

19 

-3.10 

-2.89 

-2.25 

-4.00 

,    , 

130 

20 

-2.81 

-2.78 

-2.00 

-3.50 

.    . 

79 

21 

-2.58 

-2.74 

-2.00 

-3.50 

35 

Total 

17,716 

lomorphic  conditions  of  the  color  factor  are  known,  (a)  in- 
tense pigmentation,  (6)  dilute  pigmentation,  (c)  red-eyed 
dilution,  and  (d)  Himalayan  albinism.  In  rabbits,  the  color 
factor  occurs  in  three  forms,  (a)  ordinary  pigmentation, 
(b)  Himalayan  albinism,  (c)  pure  white  albinism.  In  rats 
also  the  color  factor  occurs  in  three  forms,  but  not  the  same 


188  GENETICS  AND  EUGENICS 

three  as  in  rabbits.  They  are  (a)  ordinary  pigmentation, 
{b)  ruby-eyed  dilution  (\\Tiiting)  —  perhaps  homologous  with 
red-eyed  dilution  in  guinea-pigs  —  and  (c)  albinism. 

In  Drosophila  a  factor  for  eye-color  has  been  discovered 
in  several  allelomorphic  forms,  such  as  white,  eosin,  buff, 
cherry,  blood,  and  red,  analogous  with  the  allelomorphs  of 
the  color  factor  found  in  guinea-pigs. 

The  agouti  factor  of  rodents  occurs  in  the  rabbit  in  three 
allelomorphic  forms,  (a)  ordinary  gray,  (b)  black-and-tan, 
and  (c)  non-agouti.  In  mice  the  agouti  series  includes  (a)  or- 
dinary gray,  (h)  gray  with  white  belly,  (c)  yellow,  and  (d) 
non-agouti.  In  a  cavy  {Cavia  rufescens)  and  its  guinea-pig 
hybrids,  it  has  three  forms,  (a)  agouti  with  light  belly, 
{h)  agouti  with  ticked  belly,  and  (c)  non-agouti. 

The  extension  factor  has  in  rabbits  three  allelomorphic 
forms  (a)  ordinary  extension  as  in  gray  or  black  rabbits, 
(6)  "darkened"  extension  (DE,  Punnett)  seen  in  steel  gray 
rabbits,  and  (c)  non-extension  (restriction)  seen  in  yellow 
and  in  tortoise  rabbits.  Extension  in  guinea-pigs  assumes 
three  alternative  forms  seen  in  (a)  black,  (6)  tortoise,  and 
(c)  yellow. 

AMiite  spotting  shows  numerous  allelomorphic  forms.  In 
rats  (a)  hooded  pattern,  (6)  "Irish"  pattern,  and  (c)  self 
pattern  are  allelomorphs.  In  rabbits,  all  known  forms  of 
white  spotting  behave  as  allelomorphs.  These  include  at 
least  three  different  patterns  of  Dutch  marking  as  well  as 
English  marking.^ 

In  silkworms  Tanaka  discovered  a  series  of  three  factors 
for  marking  of  the  larva,  which  behave  as  allelomorphs, 
although  he  prefers  to  describe  them  as  factors  completely 
coupled.  The  three  are  Q  (quail),  Qs  (striped  quail)  and  Qm 
(moricaud  quail).  He  also  observed  several  minor  forms  of 
Q,  which  he  designated  QS  Q^,  Q^,  and  Q'^,  which  consider- 
ably extend  the  allelomorphic  series,  but  which  differ  so 

^  But  in  mice  two  forms  of  white  spotting  are  known  which  are  not  allelomorphic, 
nor  even  Imked.  These  are  known  as  black-eyed  white  and  piebald  respectively. 
On  the  chromosome  theory,  they  must  be  located  in  diflFerent  chromosomes. 


MODIFYING  FACTORS  189 

little  one  from  another  that  the  variation  is  practically 
continuous. 

Some  gametic  factors  show  their  influence  chiefly,  if  not 
exclusively,  m  the  form  of  changed  action  of  other  factors. 
Thus  the  ordinary  extension  factor  in  rabbits  produces  with 
the  regular  agouti  factor  an  ordinary  gray  coat,  but  the 
darkened  extension  factor  produces  with  the  same  agouti 
factor  a  steel  gray  coat.  We  think  of  the  character  of  the 
gray  marking  as  a  consequence  of  the  agouti  factor  but  find 
in  reality  that  it  is  changed  by  a  change  in  the  extension 
factor,  no  less  than  by  changes  in  the  agouti  factor.  It  is 
assumed  that  there  are  many  factors  whose  only  discoverable 
function  is  to  modify  the  action  of  other  factors  and  when 
we  find  that  some  particular  character,  manifestly  influenced 
mainly  by  a  single  gene,  has  undergone  slight  change,  or 
continues  to  change  progressively  under  continued  selection, 
it  is  safer  to  assume  that  modifying  factors  are  concerned  in 
the  matter  than  that  the  principal  gene  is  gradually  changing. 

The  substance  of  our  present  knowledge  as  to  changes  in 
genes  may  be  summed  up  in  the  statement  that  such  changes 
come  or  go  suddenly  and  in  their  entirety,  and  cannot,  so  far 
as  we  know,  be  influenced  by  selection  or  any  other  control- 
able  process.  Hence  we  may  well  call  changes  in  genes 
mutations. 


CHAPTER  XXII 

INHERITANCE  OF  SIZE  AND  OTHER  QUANTITATIVE 
CHARACTERS.    THE  HYPOTHESIS  OF  MULTIPLE  FACTORS 

Having  observed  how  wide-spread  unit-character  variations 
are  and  what  an  important  part  they  play  in  the  formation  of 
varieties  of  domesticated  animals  and  cultivated  plants,  it  is 
natural  to  inquire  whether  any  other  sort  of  heritable  varia- 
tions occur,  whether  in  the  last  analysis  all  inheritance  is 
Mendelian  inheritance.  This  view  is  held  by  many  students 
of  genetics  at  the  present  time.  The  cases  of  doubtful  inter- 
pretation relate  chiefly  to  variations  in  size  or  shape  of  the 
organism  or  of  its  parts,  cases  in  which  the  characters  under 
observation  vary  continuously. 

That  size  may  be  affected  by  ordinary  Mendelian  factors 
has  never  been  questioned.  One  of  the  seven  unit-character 
variations  studied  by  Mendel  himself  was  found  in  the  cross 
between  tall  and  short  varieties  of  peas.  Tall  was  found  to  be 
dominant  and  the  alternative  conditions,  tall  and  short,  were 
observed  to  segregate  in  true-breeding  types  in  F2.  In  man 
brachydactylism  was  early  demonstrated  to  be  a  dominant 
unit-character,  by  Farabee  confirmed  by  Drinkwater.  In 
this  peculiar  condition,  the  skeleton  is  shortened  throughout, 
and  in  particular  the  fingers  are  reduced  from  the  usual 
three-jointed  to  the  short,  two- jointed  condition.  An  analo- 
gous variation  in  Drosophila  known  as  ''dachs"  is  inherited 
in  the  "second  chromosome"  group  of  genes. 

But  the  ordinary  size  differences  between  races  of  men, 
breeds  of  animals,  or  varieties  of  plants,  are  not  inherited  in 
this  simple  way,  with  dominance  of  one  type,  followed  by 
complete  segregation  from  an  alternative  type.  As  a  rule 
intermediates  or  blends  are  produced  in  Fi  (see  Fig.  130).  In 
F2  the  commonest  type  is  still  the  intermediate  as  in  Fi,  but 
variability  is  considerably  increased,  which  may  be  regarded 

190 


Fig.  126.     Angora  male. 


Fig.  127.     Lop-eared  female. 


Fig.  128.     Fi  black  half-lop. 


Fig.  129.     F2  albino  half-lop. 


Fig.  130.    Skulls  of  mother  (at  left),  of  father  (at  right)  and  of  son  (between). 

Compare  Figs.  126-128. 


SIZE  INHERITANCE  191 

as  a  tendency  toward  segregation  of  the  original  types. 
These,  the  well  established  facts,  were  at  one  time  regarded 
as  showing  the  occurrence  of  a  distinct  type  of  inheritance 
known  as  blending,  but  at  present  we  are  inclined  to  give 
them  a  different  explanation,  the  same  in  fact  as  for  ordinar\" 
Mendelian  inheritance  except  that  several  factors,  instead  of 
one,  are  supposed  to  be  concerned  in  the  case,  and  that 
dominance  is  not  in  evidence. 

If  a  large  rabbit  is  crossed  with  a  small  one,  the  young  are 
of  intermediate  size  and  the  F2  offspring  show  no  such  segre- 
gation into  large,  small,  and  intermediate-sized  individuals 
as  a  simple  Mendelian  system  would  demand.  For  if  tlie 
size  difference  between  a  large  and  a  small  rabbit  depended 
upon  one  unit-character,  then  the  F2  animals  should  be  as 
regards  size  in  the  proportions,  one  large,  two  intennediate, 
one  small.  But  in  the  cases  thus  far  studied  all  F2  individ- 
uals are  intermediate  in  size.  A  specific  case  illustrating  the 
point  is  the  following:  A  cross  was  made  between  a  large  lop- 
eared  rabbit  and  a  small  short-eared  one.  The  fonner  was 
also  a  sooty  yellow  animal  and  short-haired  (Fig.  127);  the 
latter  an  albino  and  long-haired  (angora).  See  Fig.  126.  The 
character  of  Fi  is  shown  in  Fig.  128.  Notice  first  the  smiple 
Mendelian  behavior  of  the  color  characters  and  the  hair- 
length.  Albinism  disappeared  in  Fi,  for  all  the  Fi  aninuils 
were  black.  But  it  reappeared  in  Fo;  one  Fo  albino  is  shown 
in  Fig.  129.  Long  hair  also  behaved  as  a  Mendelian  recessive 
(as  in  guinea-pigs),  disappearing  in  Fi  but  reappearing  in  F- 
as  expected,  sometimes  in  colored  individuals,  sometimes  in 
albinos,  thus  showing  its  independent  inheritance.  The 
black  character  seen  in  the  Fi  individuals  was  received  from 
the  albino  (angora)  parent,  which  had  black  ears.  The  black 
character  (dominant  in  Fi)  was  found  in  a  majority  of  the 
F2  colored  individuals  also,  as  we  should  expect,  but  the 
yellow  character  of  the  other  grandparent  reappeared  as  a 
recessive  in  F2  in  certain  of  the  individuals.  Three  inde- 
pendent coat  characters  were  thus  Mendelizing  in  the  cross, 
viz.. 


192  GENETICS  AND  EUGENICS 

Color  dominant  over  albinism. 
Black  dominant  over  yellow. 
Short  hair  dominant  over  long  hair. 

As  regards  ear-length,  neither  dominance  nor  segregation 
of  the  difference  between  the  parents  is  observable.  All  the 
Fi  as  well  as  the  r2  individuals  have  ears  of  intermediate 
length.  The  inheritance  is  what  has  been  called  blending. 
The  same  is  true  as  regards  size  of  the  body. 

In  Fig.  130  the  skulls  of  the  parents  are  shown  with  the  skull 
of  the  Fi  individual  between  them.  In  absolute  dimensions, 
as  well  as  in  the  proportions  of  its  parts  the  Fi  skull  is  strictly 
intermediate.  The  same  blending  effect  was  observed  in  all 
other  parts  of  the  skeleton. 

The  multiple  factor  hypothesis.  It  is  clear  that  in  blending 
inheritance  there  is  no  dominance,  but  the  suggestion  has 
been  made  that  nevertheless  segregation  may  occur,  and  so 
the  inheritance  may  have  a  Mendelian  basis.  This  suggestion 
was  first  made  by  a  Swedish  plant  breeder,  Nilsson-Ehle 
(1909)  who  obtained  some  very  peculiar  inheritance  ratios  in 
crosses  of  wheat  differing  in  color  of  seed  or  of  chaff. 

When  a  variety  having  brown  chaff  is  crossed  with  one 
which  has  white  chaff,  the  hybrid  plants  are  regularly  brown 
in  Fi  and  three  brown  to  one  white  in  F2,  but  a  particular 
variety  of  brown-chaffed  wheat  gave  a  different  result.  In 
fifteen  different  crosses  it  gave  uniformly  a  close  approxima- 
tion to  the  ratio  15:1  instead  of  3:1.  The  totals  are  suffi- 
ciently large  to  leave  no  doubt  of  this.  They  are  one  thou- 
sand four  hundred  and  ten  brown  to  ninety-four  white, 
exactly  15 :1.  This  is  clearly  a  dihybrid  Mendelian  ratio,  and 
Nilsson-Ehle  interprets  it  to  mean  that  there  exist  in  this 
case  two  independent  factors,  each  of  which  is  able  by  itself 
to  produce  the  brown  coloration,  though  no  qualitative 
difference  can  be  detected  between  them. 

A  still  more  remarkable  case  was  observed  in  crosses  be- 
tween varieties  of  wheat  of  different  grain-color.  Red  crossed 
with  white  gave  ordinarily  all  red  in  Fi  and  three  red  to  one 
white  in  F2,  but  a  certain  native  Swedish  sort  gave  only  red 


HYPOTHESIS  OF  MULTIPLE  FACTORS         193 

(several  hundred  seeds)  in  F2.  This  result  was  so  surprising 
that  one  cross  which  had  yielded  seventy-eight  grains  of 
wheat  in  F2  was  followed  into  F3,  with  the  following  result: 

Expected 
50  plants  gave  only  red  seed  (being  homozygous) 37 

5  "  "     approximately  63  R  :  1  W  (being  trihybrid) 8 

15  "  "                 "              15  R  :  1  W  (being  dihybrid)   12 

8  "  "                 «                3  R  :  1  W  (being  monohybrid) 6 

0  "  "                                    all  white 1 

The  interpretation  given  by  Nilsson-Ehle  is  this.  The  red 
variety  used  in  this  cross  bears  three  independent  factors, 
each  of  which  by  itself  is  able  to  produce  the  red  character. 
Their  joint  action  is  not  different  in  kind  from  their  action 
separately,  though  possibly  quantitatively  greater.  The  F2 
generation  should  contain  one  white  seed  in  sixty-four.  It 
happens  that  none  was  obtained  in  this  generation.  The 
next  generation  should  contain,  in  a  total  of  sixty-four  indi- 
viduals, the  sorts  actually  observed  as  well  as  a  sort  which 
would  produce  only  white  seed,  the  progeny  namely  of  the 
expected  white  seed  of  F2,  but  as  that  was  not  obtained,  the 
all-white  plant  of  F3  could  not  be  obtained  either.  The  ex- 
pected proportions  of  the  several  classes  in  F3  are  given  for 
comparison  with  those  actually  obtained.  The  agreement 
between  expected  and  observed  is  so  good  as  to  make  it  seem 
highly  probable  that  Nilsson-Ehle's  explanation  is  correct. 
Corroborative  evidence  in  the  case  of  maize  has  been  obtained 
by  East,  and  in  shepherd's-purse  by  Shull. 

This  work  introduces  us  to  a  new  principle  which  has  im- 
portant theoretical  consequences.  If  a  character  ordinarily 
represented  by  a  single  unit  in  the  germ-plasm  may  become 
represented  by  two  or  more  such  units  identical  in  character, 
then  we  may  expect  it  to  dominate  more  persistently  in 
crosses,  fewer  recessives  being  formed  in  F2  and  subsequent 
generations.  Further,  if  duplication  of  a  unit  tends  to  in- 
crease its  intensity,  as  seems  probable,  then  we  have  in  this 
process  a  possible  explanation  of  quantitative  variation  in 
characters  which  are  non-Mendelian,  or  at  any  rate  do  not 
conform  with  a  simple  Mendelian  system.     Consider,  for 


194  GENETICS  AND  EUGENICS 

example,  the  matter  of  size  and  skeletal  proportions  in  rab- 
bits. It  is  perfectly  clear  from  the  experiments  described 
that  in  such  cases  no  dominance  occurs,  and  also  that  no 
segregation  of  a  simple  Mendelian  character  takes  place,  but 
it  is  possible  to  explain  the  observed  facts  by  the  combined 
action  of  several  similar  but  independent  factors,  the  new 
principle  which  Nilsson-Ehle  has  brought  forward.  This  is 
known  as  the  principle  of  multiple  factors.  Let  us  apply  such 
an  hypothesis  to  the  case  in  hand. 

Suppose  a  cross  be  made  involving  ear-lengths  of  approxi- 
mately four  and  eight  inches  respectively,  as  in  one  of  the 
crosses  made.  The  Fi  young  are  found  to  have  ears  about 
six  inches  long,  the  mean  of  the  parental  conditions,  and  the 
F2  young  vary  about  the  same  mean  condition.  If  a  single 
Mendelian  unit-character  made  the  difference  between  a 
four-inch  and  an  eight-inch  ear,  the  F2  young  should  be  of 
three  classes  as  follows: 

Classes  4  in.  6  in.  8  in. 

Frequencies  12  1 

(Compare  Fig.  131,  bottom  left,  and  Table  28.)  The  grand- 
parental  conditions  should  in  this  case  reappear  in  half  the 
young.  This  clearly  does  not  occur  in  the  rabbit  experiment. 
But  if  two  unit-characters  were  involved,  Fi  would  be  un- 
changed, all  six  inches,  yet  the  F2  classes  would  be  more 
numerous,  viz.,  four,  ^ve,  six,  seven,  and  eight  inches,  and 
their  relative  frequencies  as  shown  by  the  height  of  the 
columns  in  Fig.  131,  middle  left,  one,  four,  six,  four,  one. 
The  grandparental  states  would  now  reappear  in  one-eighth 
of  the  F2  young,  while  three-eights  would  be  intermediate. 
It  is  certain,  however,  that  in  rabbits  the  grandparental  con- 
ditions, if  they  reappear  at  all,  do  not  reappear  with  any  such 
frequency  as  this. 

If  three  independent  size-factors  were  involved  in  the  cross, 
the  Fi  individuals  should  all  fall  in  the  same  middle  group, 
as  before,  viz.,  six  inches,  but  the  F2  classes  should  number 
seven,  and  their  relative  frequencies  would  be  as  shown  in 


HYPOTHESIS  OF  MULTIPLE  FACTORS  195 

Fig.  131,  top  left.  For  four  independent  size-factors,  the  F2 
classes  would  be  more  numerous  still,  viz.,  nine  (Fig.  131, 
right),  and  the  extreme  ear-size  of  either  grandparent  would 
be  expected  to  reappear  in  only  one  out  of  two  hundred  and 
fifty-six  offspring,  while  considerably  more  than  half  of  them 
would  fail  within  the  closely  intermediate  classes  included 
between  five  and  one-half  and  six  and  one-half  inches,  the 


I 


I 


Fig.  131.  —  Diagrams  to  show  the  number  and  size  of  the  classes  of  individuals  to 
be  expected  from  a  cross  involving  Mendelian  segregation  without  dominance.  One 
Mendelian  unit  involved,  bottom  left;  two  units,  middle  left;  three  units,  top  left; 
four  units,  right. 

three  middle  classes  of  the  diagram.  With  six  size-characters, 
the  extreme  size  of  a  grandparent  would  reappear  no  oftener 
than  once  in  four  thousand  times,  while  with  a  dozen  such 
independent  characters  it  would  recur  only  once  in  some 
seventeen  million  times.  It  would  be  remarkable  if  under 
such  conditions  the  extreme  size  were  ever  recovered  from 
an  ordinary  cross. 


196 


GENETICS  AND  EUGENICS 


From  Table  28  it  will  be  seen  that  when  three  like  factors 
are  concerned,  fifty  to  one  hundred  individuals  must  be  pro- 
duced to  insure  the  recovery  of  the  parental  condition  in  F2; 


with  4  like  factors,  200-300  individuals  must  be  produced ; 
"     5    "         "       over  1000         "  "       '^  "         ;   and 

«     6    "         "  "    4000         "  "      "  «        . 


The  foregoing  calculations  are  based  on  the  assumption 
that  each  of  the  several  hypothetical  factors  involved  has  an 
equal  influence  in  determining  the  general  result  and  that  all 

TABLE   28 

Theoretical  Factorial  Composition  of  a  Population  Produced  by  a 

Cross  Involving  more  than  a  Single  Mendelian  Factor, 

Dominance  being  wanting 


Factors 

Frequencies  of  F2  Classes 

Total 

(=4°) 

Number 

of  Homo- 

zygotes 

(=2°) 

Per  Cent 

of  Homo- 

zygotes 

1 

1 

2 

1 

4 

2 

50.0 

2 

,    , 

,    , 

,    , 

1 

4 

6 

4 

1 

.    . 

,    , 

,    , 

16 

4 

25.0 

3 

,    , 

^    , 

1 

6 

15 

20 

15 

6 

1 

,    , 

,    , 

64 

8 

12.5 

4 

,    , 

1 

8 

28 

56 

70 

56 

28 

8 

1 

,    , 

256 

16 

6.2 

5 

1 

10 

45 

120 

210 

252 

210 

120 

45 

10 

1 

1024 

32 

3.1 

6 

1 

12 

66 

220 

495 

792 

924 

792 

495 

220 

66 

12 

1 

4096 

64 

1.5 

are  mutually  independent  (not  linked).  If,  however,  one  or 
more  of  the  factors  had  greater  influence  than  the  others,  the 
apparent  blending  would  be  less  perfect  and  a  "tendency 
toward  segregation"  or  "imperfect  segregation"  would  re- 
sult. It  is  probable  that  this  is  the  correct  explanation  of 
what  at  one  time  was  called  "a  type  of  inheritance  inter- 
mediate between  Mendelian  and  blending."  Also  if  certain 
of  the  multiple  factors  were  linked  (borne  in  the  same  chro- 
mosome), this  would  result  in  a  tendency  of  the  factors  to 
segregate  in  groups  as  originally  introduced  into  the  cross, 
although  crossing-over  might  lead  to  the  production  of 
transitional  types,  any  of  which  would  "breed  true"  as  soon 
as  all  factors  involved  became  homozygous. 


SIZE  INHERITANCE  197 

A  considerable  number  of  cases  of  size  inheritance  has  now 
been  studied  in  both  animals  and  plants.  Their  results  may 
be  summarized  thus:  (1)  When  animals  or  plants  are  crossed 
which  have  racial  differences  in  size  or  other  characters,  in 
respect  to  which  each  race  shows  continuous  variation  about 
a  different  mean,  the  Fi  progeny  are  of  intermediate  size.^ 
They  may  or  may  not  be  more  variable  than  the  races 
crossed,  but  quite  commonly  are  not.  (2)  The  F2  generation 
as  a  whole  commonly  varies  about  the  same  intermediate 
mean  as  the  Fi  generation,  but  its  variability  as  measured  by 
the  standard  deviation  or  the  coefficient  of  variation  is  usu- 
ally greater  than  that  of  the  Fi  generation.  The  increased 
variability  of  F  2  as  compared  with  Fi  may  in  extreme  cases 
include  forms  larger  than  the  larger  parental  race  or  smaller 
than  the  smaller  race,  and  which  show  a  tendency  to  vary 
in  F3  about  the  same  size  as  characterized  the  F2  parent. 

Some  illustrative  cases  may  be  cited.  Phillips  (1912,  1914) 
crossed  two  breeds  of  ducks  which  differed  markedly  in  size, 
namely  Rouens  and  Mallards.  The  average  adult  weight  of 
the  Rouen  race  used  w^as,  for  males,  two  thousand  three  hun- 
dred grams,  and  for  females  two  thousand  two  hundred  and 
thirty-seven  grams.  Corresponding  weights  for  the  Mallard 
race  were  one  thousand  sixty-eight  and  nine  hundred  and 
twenty-eight  grams  respectively.  The  Rouens  accordingly 
were  more  than  tw^ice  as  large  as  the  Mallards.  The  two 
races  did  not  overlap  in  weight,  as  appears  from  Table  29, 
where  the  animals  are  classified  by  weight.  In  this  table  the 
mean  weight  of  the  Mallards  is  taken  as  the  center  of  class 
2  and  the  mean  weight  of  the  Rouens  as  the  center  of  class  10. 
Each  sex  was  classified  separately  but  the  two  are  combined 
in  classes  bearing  the  same  class  number  in  Table  29.  The 
seventy  Fi  offspring  have  their  mode  in  the  intermediate 

1  I  leave  out  of  consideration  here  such  differences  as  exist  between  till  ami  dwarf 
peas,  and  between  brachydactylous  and  normal  men.  In  such  cases  a  simple 
Mendelizing  difference  exists,  which  shows  both  dominance  and  segregation  in 
typical  fashion.  Aside  from  this  simple  difference,  however,  ordinary  size  differ- 
ences exist  in  such  cases,  which  I  doubt  not  follow  the  ordinary  rules  of  size  in- 
heritance. 


198  GENETICS  AND  EUGENICS 

class  6,  though  they  range  all  the  way  from  class  2  to  class  9. 
The  sixty-three  Fa  offspring  likewise  have  their  mode  in 
class  6,  and  are  slightly  more  variable  than  Fi,  though  only 
one  aberrant  individual  falls  beyond  the  range  of  Fi.  This  is 
a  case  in  which  apparently  many  independent  factors  of 
approximately  equal  influence  on  weight  are  concerned  and 
which  do  not  segregate  in  linked  groups.  The  result  is  that 
both  Fi  and  F2  vary  symmetrically  about  the  same  strictly 
intermediate  mode  (class  6). 

A  case  in  which  fewer  factors  are  involved  or  in  which  the 
factors  are  either  not  all  of  equal  influence  or  occur  in  linked 
groups  is  the  following.  Punnett  and  Bailey  crossed  two 
breeds  of  fowls  differing  w^idely  in  weight,  the  larger  breed 
being  represented  in  a  gold-penciled  Hamburg  cock,  the 
smaller  in  silver  Sebright  bantam  hens.  The  relative  size 
of  the  breeds  is  shown  in  Table  29a.  As  male  fowls  are  larger 
than  females,  the  weight  of  each  sex  is  tabulated  separately 
in  absolute  weight  units  (grams).  The  weight  of  the  Fi  birds 
was  much  nearer  that  of  the  larger  than  that  of  the  smaller 
parent  breed,  an  indication  that  one  or  more  of  the  factors 
for  large  size  show  dominance.  An  alternative  interpretation 
would  ascribe  the  large  size  of  Fi  to  hybrid  vigor.  (See 
Chapter  XXVII.)  Possibly  each  explanation  is  in  part 
correct.  The  F2  generation  showed  very  great  variability  in 
weight,  covering  the  ranges  of  both  parent  breeds,  so  far  as 
those  ranges  had  been  ascertained  for  the  material  studied. 
But  the  variation  curve  for  F2  was  not  symmetrical  about  an 
intermediate  mode,as  in  the  case  of  ducks  studied  by  Phillips. 
The  mode  was  close  to  the  Fi  mode,  but  the  variation  was 
very  '*skew,"  ending  abruptly  above,  but  sloping  gradually 
downward  to  bantam  size.  When  the  more  extreme  F2  in 
dividuals  were  mated,  large  with  large  and  small  with  small, 
broods  were  obtained  which  averaged  larger  than  pure  Ham- 
burgs  and  smaller  than  pure  Sebrights  respectively. 

Punnett  interprets  the  case  as  involving  four  independent 
factors  having  among  themselves  unequal  influence  on  the 
total  weight.   He  supposes  that  three  of  the  four  factors  are 


SIZE  INHERITANCE 


199 


TABLE  29a 

Weight  Inheritance  in  Fowls 
After  Punneft  and  Bailey  (1914) 


Weight  classes  in  grams 

500- 

600- 

700- 

800-  900-  1000-1100-  1200- 

1         '         1 

1300-  1400-  1500-1600- 

Females,  Hamburg. .  .  . 
Sebright 

1 
3 

3 

1 

17 
1 

25 
4 

27 
7 

6 
36 

1 

•    * 

15 

3 

1 
8 
3 

26 
1 

1 

4 
5 

5 
19 

2 

1 
2 

2 

29 

4 

2 

•  • 

1 
1 
9 

2 

•  • 

2 
1 

•    • 

2 

•  • 

Fi 

F2 

F3  (from  largest  Fzs)..  . . 
F3  (from  smallest  F2S) .  . 

Males,  Hamburg 

Fi 

F2 

*    * 

F3  (from  largest  F2S)  . .  . 
F3  (from  smallest  F2S)  . . 

2 

1 

borne  by  the  Hamburg  race,  and  one  by  the  Sebright  race. 
Recombinations  which  inchide  all  four  factors  produce  a  race 
larger  than  Hamburg,  seen  in  the  Fss  from  largest  F2S. 
Recombinations  which  include  the  four  allelomorphs  of 
these  factors  produce  a  race  smaller  than  Sebright,  seen  in 
the  Fgs  from  smallest  FsS,  Table  29a.  He  supposes  further 
that  two  of  the  four  hypothetical  factors  exert  a  greater  in- 
fluence than  the  other  two  on  the  total  size,  the  influence  of 
the  first  being  to  the  second  as  66  to  30.  The  figures  are  purely 
provisional  and  are  intended  to  indicate  a  form  of  explanation 
which  may  cover  such  cases  satisfactorily.  But  it  must  be 
confessed  that  the  number  of  individuals  studied  bv  Punnet t 
and  Bailey  is  small  and  their  assumptions  as  to  the  number 
and  potency  of  the  hypothetical  factors  is  quite  arbitrary. 

The  extensive  and  carefully  executed  studies  of  Emerson 
and  East  (1913)  upon  crosses  of  maize  involving  difl'erences 
in  size  and  other  quantitative  characters  afford  excellent  il- 
lustrations of  the  usual  consequences  of  size  crosses.  The 
simplest  and  clearest  cut  cases  relate  to  the  size  of  the  ear  or 
of  the  seeds  borne  upon  it.  The  behavior  of  ear-diameter  in 
crosses  is  shown  in  Table  30. 


200  GENETICS  AND  EUGENICS 

Both  Fi  and  F2  are  intermediate  in  character  in  comparison 
with  the  parent  races,  but  F2  is  shghtly  more  variable.  Dif- 
ferent lots  of  Fi  progeny  (combined  in  Table  30)  give  co- 
efficients of  variability  of  8.29  and  6.88  respectively,  whereas 
F2  progeny  have  coefficients  ranging  from  9.66  to  11.77.  The 
extreme  ranges  of  the  parent  races  are  not  attained  in  F2. 
This  case  is  similar  to  that  of  weight  inheritance  in  ducks, 
except  that  F  2  is  on  the  average  less  than  Fi,  being  more 
nearly  intermediate  between  the  parental  races  than  was  Fi. 
The  case  is  probably  complicated  by  hybrid  vigor  in  Fi, 
which  is  not  retained  in  F2.  (See  Chapter  XXVII.)  It  is 
evident  that  so  many  independent  factors  are  involved  that 
no  complete  segregation  occurs  in  F2. 

Table  31  shows  the  result  of  crossing  two  races  of  com  (A 
and  B)  differing  in  seed  width.  In  this  cross  also,  Fi,  and  F2 
were  alike  intermediate,  but  the  latter  was  slightly  more 
variable.  It  was  found  that  the  F2  plants  differed  in  genetic 
character  as  to  seed  width.  An  F2  with  low  seed  width  (143 
mm.)  produced  an  F3  likewise  low  (mean  141.3  mm.);  and 
F2  with  seed  width  above  the  average  (178  mm.)  produced 
an  F3  of  like  character  (mean  172.9  mm.).  The  range  of  the 
low  selected  F3  extended  even  lower  than  the  range  of  the 
uncrossed  low  race  (B),  which  is  similar  to  the  result  obtained 
by  Punnett  and  Bailey  in  the  weight  inheritance  of  fowls  and 
suggests  a  similar  explanation,  recombination  of  factors. 

Some  instructive  cases  involving  multiple  factors  affecting 
the  size  and  shape  of  fruits  have  been  studied  by  Gross. 
See  Fig.  132.  It  is  evident  that  in  these  cases  length  and 
width  of  the  fruit  are  affected  by  numerous  independent 
factors  which  recombine  so  as  to  form  a  complete  series  of 
intergrading  forms. 

In  garden  peas  the  time  between  germination  of  the  seed 
and  flowering  varies  greatly  in  different  varieties.  In  early 
varieties  the  time  is  short,  in  late  varieties  it  is  relatively  long. 

Hoshino  crossed  two  varieties  of  garden  peas  which  had 
been  found  to  breed  very  true  as  to  flowering  time  and  flower 
color.    One  variety  was  early  and  white  flowered,  the  other 


SIZE   INHERITANCE 


^201 


was  late  and  red  flowered.  Fi  was  very  uniform  also,  bein^ 
red  in  flower  color  and  nearly  as  late  in  flowering  as  the  late 
parent.  F2  showed  regular  Mendelian  segregation  as  to  flower 
color  into  three  reds  to  one  white.   As  regards  time  of  flower- 


TABLE   29 

Weight  Distribution  of  Ducks 

After  Phillips  {1912  and  1914) 


Class  Number 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

Rouen 

3 

3 

9 

5 

1 

1 

Mallard 

23 

54 

23 

2 

.  . 

.     . 

.    , 

.   . 

.   . 

. . 

.    . 

,    , 

,  , 

Fi 

,   . 

1 

1 

4 

21 

35 

6 

1 

1 

. . 

,    . 

•    • 

.  • 

F2 

•   • 

1 

6 

10 

22 

13 

9 

1 

1 

TABLE  30 

Ear-Diameter  in  Crosses  of  Maize 

After  Emerson  and  East  (1913,  p.  56) 


Class 
means 

22.5 

24.5 

26.5 

28.5 

30.5 

32.5 

34.5 

36.5 

38.5 

40.5 

42.5 

44.5 

46.5 

48.5 

50.5 

52.5 

54.5 

56.5 

in  mm. 

Race  A . . 

.. 

2 

2 

2 

4 

2 

4 

5 

2 

RaceB.. 

15 

32 

14 

3 

1 

,    , 

,    , 

.  . 

.    . 

.  . 

. . 

.  . 

.  . 

.  • 

• . 

•  • 

Fi    

1 
35 

2 
38 

4 
69 

8 
50 

11 
43 

21 
30 

12 
17 

6 

8 

1 

•  • 

F2 

2 

6 

23 

Class  means 

in  mm.  (25 

seeds) 


Race  A  . . . . 

Race  B 

Fi 

F2 

F3  (from  Fi, 

class  143) 
F3  (from  F2, 

class  178) 


113 


118 


TABLE  31 

Width  of  Seed  in  Crosses  of  Maize 

After  Emerson  and  East  (1913,  p.  63). 


123 


128 


133 


10 


138 


4 

5 

13 


143 


7 
15 


148 


1 

i2 

9 

1 


153 


1 

10 

8 
1 


158 


1 
31 

6 

5 


163 


2 
23 


168 


3 
24 

1 

5 


173 


2 
15 


178 


4 
26 


10 


183 


3 
12 


13 


188 


6 
12 


193 


198'203  208  213  218  223  228,233  238 


202 


GENETICS  AND  EUGENICS 


ing,  F2  was  intermediate  but  highly  variable,  covering  prac- 
tically the  entire  range  from  the  flowering  time  of  the  early 
to  that  of  the  late  parent.  F3  was  also  highly  variable  but  a 
few  families  were  found  to  be  as  ''constant "  in  flowering  time 
as  the  parent  varieties,  and  in  r4  the  proportion  of  constant 
families  had  increased  further.    Two  hundred  and  thirty  of 


Fig.  132.  A  cross  of  two  varieties  of  peppers  differing  greatly  in  size  and  shape  of  fruits.  Fruits  of 
the  parent  varieties  are  shown  at  P  and  P,  of  Fi  between  them,  and  of  F2  in  the  four  lower  rows. 
Each  fruit  is  taken  from  a  diflFerent  plant  and  is  typical  for  the  plant.     (After  Gross.) 

the  four  hundred  and  twenty-one  F4  famihes  studied  by 
Hoshino  were  found  to  be  as  "constant"  in  flowering  time 
as  the  parent  varieties.  The  mean  flowering  time  in  days 
from  sprouting  as  observed  by  Hoshino  is  given  in  Table  32. 
It  will  be  observed  that  the  white-flowered  F4  constant 
families  were  all  early  or  intermediate  in  flowering  time 
whereas  the  red-flowered  families  were  chiefly  late.  This 
clearly  indicates  linkage,  or  coupling,  between  flower  color 
and  time  of  flowering.  But  flower  color  clearly  Mendelizes, 
hence  flowering  time  must  also  depend  upon  a  Mendelizing 
gene,  which  is  linked  with  the  gene  for  red  flower  color. 


GAMETIC  PURITY 


203 


Cross  overs  occasionally  occur  resulting,  for  example,  in  the 
F4  pure  red  early  family  shown  in  Table  32.  But  if  these  two 
were  the  only  genetic  factors  involved  in  the  cross,  no  *' con- 
stant" families  of  intermediate  flowering  time  could  result 

TABLE  32 

Variation  in  flowering  time  of  two  pure  varieties  of  garden  peas,  one  Early 
White,  the  other  Late  Red;  and  a  classification,  both  as  to  color  and  as  to  flowering 
time,  of  two  hundred  and  thirty  F4  families  produced  by  crossing  the  two  varieties, 
these  F4  families  being  all  regarded  as  "  constant  "  in  flowering  time  because  of 
their  low  variability,  as  low  as  that  of  the  parent  varieties.  Only  the  position  of 
the  mean  of  each  r4  family  is  given  in  the  table,  not  its  range  as  in  the  case  of  the 
parent  varieties. 


Days  to  Flowering 

32 

33 

34  35 

36 

37  38  39  40  41  42  43  44  45  46  47  48 

Early  White  Parent   . .  . 
T^ate  Hed  Parent         .  .  . 

1 

2 

11     7 

9 

13     7   ..      1 .. 

F4  White  Families    .... 
F4  Mixed  Families, 
White  or  Red    

•   • 

1  13 
..      1 

5 

1 

2   . .     3  18  12  15  14  13  12     3   . .    . . 
1   ..      1     1   ..      1     1     4     2   

Ya  Red  Families    

..     1 

1     1     3  12  13     4     6     2    . . 

Total    "  Constant "    F4 
Families        

1  15 

6 

3           4  20  13  19  27  30  18     9     2 

25 

142 

Days  to  Flowering 

49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64 

Earlv  White  Parent 

Late  Red  Parent 

5  11  10  11  10     7  10     1    ..     3     2     1 

Va  W^Viitp  T^^amilips 

F4   Mixed   Families,  White 

.      1     3 

Ya  Red  Families 

5     3     3     2  10  13     3  10     8     1     1   

Total  "  Constant  "  F4 
Families 

5     3     3     2  11  16     3  10    8     1     1   

-^ 

63                                            =  230 

from  the  cross.  As  a  matter  of  fact  more  than  half  the  F4 
families  are  of  this  constant  intermediate  type,  which  shows 
that  one  or  more  other  factors,  independent  of  the  chief 
factor  for  flowering  time,  must  be  concerned  in  the  result. 
Hoshino  supposes  that  a  single  supplementary  factor  (not 


204  GENETICS  AND  EUGENICS 

linked  with  flower  color  or  with  the  chief  factor  for  flowering 
time)  will  account  for  the  case.    In  accordance  with  this 
view,  four  true-breeding  combinations  of  the  factors  for 
flowering  time  might  be  expected,  and  it  is  possible  that  their 
modes  fall  in  Table  32  on  35  days,  40  days,  44  days,  and  54 
days  respectively,  all  of  which  show  high  frequencies.    An- 
other possibility  is  that  several  modifying  factors  acting  in 
various  combinations  produce  the  wide  ranging  group  of  142 
** constant"  intermediate  families  and  that  linkage  among 
these  modifying  factors  is  responsible  for  the  apparent  dis- 
continuity between  the  intermediate  and  the  early  and  the 
late  groups.     Certainly  more  than  one  supplementary  or 
modifying  factor  is  in  evidence.   For  it  is  to  be  remembered 
that  in  Table  32,  the  F4  distribution  is  not  that  of  individual 
plants  varying  round  particular  modes,  but  each  frequency 
indicated  is  itself  the  mode  of  a  family,  **  constant  as  to  that 
particular  modal   length   of   time  between   sprouting   and 
flowering.    Accordingly  the  ''constant"  varieties  resulting 
from  the  cross  are  not  four  only,  as  a  two  factor  scheme  would 
demand,  but  their  number  is  very  great,  since  they  range 
with  only  two  apparent  breaks  all  the  way  from  the  original 
early  to  the  original  late  variety.    Such  a  result  could  be 
produced  only  by  numerous  modifying  factors,  which  in 
action  supplement,  or  else  inhibit,  the  action  of  the  chief 
gene  for  flowering  time  so  clearly  linked  with  red  flower 
color  in  transmission.     No  other  "factorial"  explanation 
seems  admissible. 


CHAPTER  XXIII 

GENETIC  CHANGES  AND  THE  CHROMOSOMES 

In  one  way  our  views  concerning  heredity  have  been  consid- 
erably simplified  by  the  discovery  that  blending  inheritance 
may  be  included  in  the  category  of  Mendelian  inheritance. 
One  mechanism  will  now  suffice  for  all  kinds  of  inlicri- 
tance,  this  mechanism  being  found  in  the  chromosomes. 
In  them,  we  may  reasonably  suppose,  is  found  the  material 
basis  of  every  inherited  character.  When  the  inheritance 
is  of  the  simplest  kind,  involving  presence  or  absence  of 
color  or  some  similar  character,  we  assume  that  a  genetic 
change  has  occurred  in  a  single,  definite  locus  in  a  particular 
chromosome,  and  that  this  single  change  is  responsible  for 
the  observed  inherited  variation.  Other  characters  depend 
on  two  or  more  genes,  which  may  lie  at  different  loci  in  the 
same  chromosome,  or  even  in  different  chromosomes.  Thus 
the  gray  coat  of  a  rabbit  is  an  inherited  character  which  de- 
pends on  at  least  five  different  genes,  each  of  which  appar- 
ently lies  in  a  different  chromosome.  These  are  (1)  a  color 
factor,  (2)  a  black  factor,  (3)  an  extension  factor,  (4)  an 
agouti  factor  and  (5)  an  intensity  factor.  Each  of  these 
factors  or  genes  behaves  as  an  independent  unit  in  trans- 
mission. We  know  of  their  existence  only  because  each  of 
them  has  been  observed  to  occur  in  two  or  more  alternative 
forms.  For  a  gene  which  remains  unchanged  remains  un- 
known. We  do  not  know  how  many  undiscovered  genes  are 
concerned  in  producing  the  gray  coat  of  a  rabbit,  nor  in  what 
linkage-systems  (chromosomes)  they  he.  These  few  have 
revealed  themselves  by  their  striking  variations.  By  various 
combinations  of  the  different  forms  of  these  five  genes,  we 
get  all  the  known  color  varieties  of  gray,  black,  yellow,  and 
white  rabbits.  When  it  comes  to  the  inheritance  of  size 
differences  among  rabbits,  we  suppose  that  genes  affecting 

205 


206  GENETICS  AND  EUGENICS 

size  are  involved  and  that  they  also  are  located  in  the  chro- 
mosomes. But  it  is  clear  that  the  size  genes  must  be  numer- 
ous since  the  inheritance  of  size  is  blending,  and  they  are 
probably  located  in  many  different  chromosomes  or  even  in 
all  the  different  chromosomes.  That  genes  do  affect  size  is 
shown  by  the  t;yT)ical  Mendelian  behavior  of  the  characters 
tall  and  short  in  crosses  of  peas,  and  brachydactyl  and  nor- 
mal in  human  families.  The  case  studied  by  Hoshino  in 
which  late  flowering  in  peas  was  found  to  be  coupled  with 
red  flow^er  color  is  important  because  it  shows  that  a  gene 
which  affects  a  quantitatively  varying  character,  one  which 
blends  in  heredity,  is  located  in  the  same  chromosome  with  a 
color  gene.  There  is  no  reason  to  think  that  any  genes  occur 
elsewhere  in  the  gamete  than  in  the  chromosomes. 

An  apparent  exception  occurs  in  the  case  of  a  plant, 
Mirabilis,  the  cultivated  four-o-clock,  studied  by  Correns. 
A  single  plant  of  this  species  arose  in  his  cultures,  which  had 
white-margined  leaves,  the  white  areas  being  due  to  an  ab- 
normal condition  of  the  normally  green  plastids,  which  are 
cytoplasmic  (not  nuclear)  structures.  Sometimes  entire 
branches  arose  on  this  plant  (or  its  descendants)  which  were 
white,  and  which  contained  only  colorless  plastids,  and  others 
which  were  green  containing  normal  colored  plastids.  White 
branches  produced  only  white  seedlings  when  self-pollinated 
and  green  branches  produced  only  green  seedlings.  When 
flowers  on  the  two  sorts  of  branches  were  intercrossed,  the 
seeds  borne  on  white  branches  still  produced  only  white 
seedlings,  and  those  borne  on  the  green  branches  produced 
only  green  seedlings,  which  thereafter  bred  true.  The  white 
seedlings  perished  because  without  chlorophyl  they  could  not 
live.  The  case  has  been  explained  as  one  in  which  inheritance 
is  exclusively  maternal,  by  means  of  the  ^gg  but  not  by 
means  of  the  pollen.  Further  it  is  a  cytoplasmic  structure, 
the  plastids  of  the  egg,  which  determine  the  plastid  character 
of  the  offspring,  the  nucleus  not  being  concerned  in  the  proc- 
ess. Here  then  we  seem  to  have  a  case  of  cytoplasmic  in- 
heritance in  which  nuclear  genes  are  not  concerned.    But  a 


GENETIC  CHANGES  207 

more  careful  study  of  the  case  makes  it  seem  probable  tliat 
we  are  here  dealing  with  a  pathological  condition  of  the 
cytoplasm  rather  than  with  true  inheritance.  Consider  a 
similar  case  in  animals.  The  organism  which  produces  Texas 
fever  in  cattle  is  introduced  into  the  blood  of  cattle  b\'  tlic 
bite  of  a  diseased  tick.  Among  ticks,  the  disease  passes  from 
mother  to  offspring  in  the  cytoplasm  of  the  egg.  The  sperm  is 
too  small  to  carry  the  disease  germ,  and  so  the  disease  does 
not  pass  from  father  to  offspring  in  the  sperm.  In  reciprocal 
crosses  between  diseased  and  healthy  ticks,  if  such  could  be 
made,  we  should  observe  exactly  the  same  mode  of  trans- 
mission as  in  the  four-o-clock  crosses  between  white  branches 
and  green  branches.  The  offspring  would  always  show  the 
condition  of  the  mother,  never  that  of  the  father.  But  we 
should  hesitate  to  describe  the  transmission  of  a  foreign  or- 
ganism in  the  egg  of  a  tick  as  inheritance,  and  the  same 
hesitancy  should  be  shown  regarding  the  transmission  of 
diseased  chloroplastids  in  the  cytoplasm  of  Mirabilis} 

Leaf -variegation,  quite  similar  in  appearance  to  that  just 
described,  but  of  truly  genetic  origin,  occurs  also  in  Mirabilis 
and  was  studied  simultaneously  by  Correns.  This  is  trans- 
mitted alike  in  egg-cell  and  pollen,  as  a  recessive  character, 
which  shows  that  the  gene  concerned  is  probably  borne  in 
the  nucleus.  In  certain  other  plants  (Antirrhinum,  Melan- 
drium)  leaf -variegation  is  a  dominant  character  transmitted 
equally  by  both  sexes.  In  fact,  in  a  great  majority  of  cases, 
variegation  is  inlierited  as  an  ordinary  Mendelian  character, 
either  dominant  or  recessive,  and  so  may  be  explained  as 
due  to  genes  contained  in  the  nucleus.  The  exceptional 
cases  are  cases  of  cell  pathology  rather  than  of  inheritance. 

If  then,  as  seems  probable,  genes  located  in  the  chromo- 
somes constitute  the  sole  vehicle  of  inheritance,  it  follows 
that  heritable  variations  can  arise  only  from  changes  in  the 
genes.  Such  changes  are  called  *' mutations,"  of  which  we 
can  distinguish  the  following  varieties: 

1  Similar  cases  of  "maternal  inheritance"  have  been  studied  by  Baur  in  Antirrhi- 
num, by  Gregory  in  Primula,  by  Ikeno  in  Plantago,  and  by  Winge  in  Hunmlus. 


208  GENETICS  AND  EUGENICS 

(1)  Change  in  a  single  gene,  ordinary  unit-cliaracter  varia- 
tion, mutation  in  the  sense  of  Johannsen  and  Morgan. 

(2)  Doubling  of  the  normal  chromosome  number,  pre- 
sumably resulting  in  a  duplication  of  every  gene  of  the 
normal  gamete,  the  duplicate  condition  being  handed  on  per- 
manently from  generation  to  generation.  This  is  the  '*gigas  " 
type  of  mutation  first  observed  by  De  Vries  in  the  case  of 
Oenothera,  later  in  Primula  by  Gregory,  and  in  the  night- 
shade and  the  tomato  by  Winkler. 

(3)  Addition  of  a  single  extra  chromosome  to  the  regular 
number  in  the  gamete,  probably  by  duplication  of  a  single 
chromosome.  This  is  the  "lata"  type  of  mutation  as  ob- 
served in  Oenothera,  and  it  is  related  to  the  phenomenon  of 
non-disjunction  as  observed  by  Bridges  in  the  case  of  the 
sex-chromosome  in  eggs  of  Drosophila. 

(4)  Loss  of  a  definite  part  of  the  sex-chromosome  of  Dro- 
sophila, has  been  described  by  Bridges  under  the  name  ''de- 
ficiency." This  involved  the  simultaneous  disappearance 
from  a  single  chromosome  of  at  least  two  neighboring  genes. 

Of  these  four  varieties  of  mutation,  the  last  two  may  be 
regarded  as  rare  and  more  or  less  pathological  phenomena, 
the  second  leads  occasionally  to  the  sudden  origin  of  a  new 
variety  of  flowering  plant,  and  may  have  functioned  in  the 
evolution  of  some  of  the  lower  plants  (mosses,  algae)  and 
lower  animals,  but  beyond  a  doubt  the  first  mentioned  vari- 
ety of  mutation,  spontaneous  change  in  single  genes,  is  the 
usual. and  continuously  operative  method  by  which  genetic 
changes  arise  in  both  animals  and  plants.  To  this  we  must 
look  for  that  unceasing  variability  of  organisms  which  fur- 
nishes the  material  for  natural  selection  to  operate  upon  and 
for  men  to  work  with  in  the  improvement  of  the  domestic 
animals  and  cultivated  plants. 


CHAPTER  XXIV 

GENETIC  CHANGES  IN  ASEXUAL  REPRODUCTION  IN 
PARTHENOGENESIS,  AND  IN  SELF-FERTH^IZATION 

The  frequency  of  occurrence  of  variation  in  single  genes  ap- 
parently is  very  different  in  different  species  of  animals  and 
plants,  and  in  different  modes  of  reproduction.  It  is  sup- 
posed to  be  commonest  in  organisms  which  reproduce  only 
sexually  but  it  must  be  remembered  that  sexual  reproduction 
favors  the  spread  of  any  genetic  change  which  happens  to 
occur,  whereas  under  asexual  reproduction  a  mutation,  how- 
ever favorable,  has  no  chance  to  spread  from  the  family  in 
which  it  originated  to  others  of  the  same  species.  Accord- 
ingly mutation  (in  single  genes)  may  seem  to  be  less  common 
than  it  really  is,  in  organisms  which  are  propagated  asexu- 
ally.  It  is  only  w^hen  systematic  search  is  made  for  genetic 
variations  that  we  gain  any  adequate  idea  of  how  com- 
monly they  occur.  Jennings  was  the  first  to  take  this  matter 
up  in  connection  with  the  asexual  reproduction  of  the  pro- 
tozoan, Paramecium.  He  was  unable  to  detect  any  genetic 
changes  in  size  in  races  of  Paramecium  reproducing  by  fission, 
but  in  a  soft-bodied  animal  like  Paramecium  in  which  body 
size  is  constantly  changing,  measurement  of  size  is  not  an 
easy  matter.  Later  Jennings  sought  more  favorable  ma- 
terial for  study  and  apparently  found  it  in  a  shelled  proto- 
zoan, Difflugia.  This  has  a  definiteness  and  rigidity  of  form 
which  is  wanting  in  Paramecium.  Its  shell  can  be  measured 
with  great  exactness  and  the  number  of  spines  which  it  bears 
can  be  counted,  and  their  length  measured.  In  the  case  of 
Difflugia  Jennings  found  that  differences  in  size,  number  of 
spines,  and  length  of  spines  may  be  observed  among  the 
asexually  produced  descendants  of  a  single  individual  tliat 
in  consequence  of  selection  these  differences  become  strength- 
ened and  divergent  races  are  thus  created.    Hegner  has  ob- 

209 


210  GENETICS  AND  EUGENICS 

served  the  occurrence  of  similar  genetic  changes  in  Arcella. 
It  is  evident  that  on  the  theory  that  genes  are  the  exclusive 
vehicles  of  inheritance,  it  must  be  supposed  that  genes  are 
undergoing  change  rather  frequently  in  the  asexual  repro- 
duction of  Difflugia  and  Arcella. 

In  the  asexual  reproduction  of  plants  genetic  changes 
known  as  bud-variations  occasionally  occur.  East  (1910) 
has  observed,  in  the  reproduction  of  the  potato  by  tubers, 
changes  in  the  shape,  color,  or  depth  of  eyes  of  the  tubers, 
such  as  are  known  to  behave  as  simple  unit-character  varia- 
tions in  reproduction  by  seed.  It  seems  probable  therefore 
that  they  have  arisen  as  changes  in  single  genes  occurring  in 
asexual  reproduction.  In  the  propagation  by  budding  of 
citrous  fruits  and  of  prunes,  according  to  Shamel,  genetic 
changes  of  commercial  importance  occur  with  so  great  fre- 
quency that  it  seems  desirable  to  take  budding  stock  only 
from  carefully  selected  trees  within  the  variety.  The  varia- 
tions noted  affect  especially  the  shape  and  size  of  the  fruit, 
or  the  vigor  and  productiveness  of  the  tree.  Shamel  de- 
scribes thus  the  recent  origin  of  a  new  and  improved  variety 
of  the  French  prune.        (See  Fig.  133.) 

In  1904,  in  a  French  prune  tree  growing  in  an  orchard  near  Saratoga, 
Cat,  one  branch  high  up  in  the  tree  was  found  bearing  very  large  fruits. 
There  is  no  question  as  to  its  being  a  true  bud  variation.  Several  grafts 
were  secured  from  this  branch  and  placed  in  bearing  peach  trees  in  order 
to  secure  early  evidence  as  to  whether  this  variation,  or  bud  sport,  could 
be  propagated.  The  fruits  produced  by  these  grafts  were  found  to  be  iden- 
tical to  those  borne  by  the  original  branch.  The  large  fruits  possessed  all 
of  the  desirable  characteristics  of  the  smaller  fruits  of  the  ordinary  French 
prune  and,  in  addition,  possessed  the  desired  improvement  in  size. 

In  order  to  give  this  strain  a  commerical  test  Mr.  Coates  bought  10 
acres  containing  about  1000  peach  trees  for  experimental  trials  of  the 
large  prune  variety.  These  trees  were  five  years  old  in  1914  at  the  time 
of  their  purchase.  The  large-fruited  French  prune  variety  was  budded 
into  every  other  row  of  the  peach  trees  with  the  usual  method  practiced 
in  top-working  citrus  and  other  fruit  trees. 

The  top-worked  trees  with  the  improved  French  prime  strain,  called 
No.  1418  for  convenience  during  the  experimental  stages,  are  in  alternate 
rows  with  the  ordinary  or  other  selected  strains  of  the  parent  variety. 
In  other  words,  in  the  10-acre  experimental  orchard  there  is  one  row  of 
No.  1418  followed  by  a  row  of  the  parent  variety,  and  so  on  throughout 


Fig.  133.  Origin  of  a  new  and  improved  variety  of  French  prune  by  a  bud-variation.  Top  row,  leaves, 
fruit  and  seed  of  the  new  variety;  bottom  row,  leaves,  fruit  and  seed  of  the  parent  variety,  shown  ou 
the  same  scale.     (After  Shamel.) 


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Fig.  134.  Eight  different  types  of  variegated  seeds  of  maize,  which  behave  in  general  as  allelomorphs 
One  to  another.  But  mutation  from  one  type  to  another  is  common,  only  the  end  types  of  the  scries, 
A  and  H,  being  as  stable  as  ordinary  Mendelian  allelomorphs.     (After  Emerson.) 


Fig.  135.  Bud  variation  (mutation)  in  a  plant 
of  maize.  The  lower  ear  bears  chiefly  light 
colored  seeds,  the  upper  ear  bears  chiefly  dark 
colored  seeds.  Each  kind,  as  a  rule,  reproduces 
its  own  sort.    (After  Emerson.) 


Fig.  136.  Bud  variation  (mutation)  in  single  ears 
of  maize.  Each  ear  bears  patches  of  very  dark 
seeds  among  the  generally  light  colored  s«H'd.s. 
Dark  seeds  and  light  seeds  reproduce  each  its 
own  sort,  as  a  rule.     (After  Emersoa.) 


GENETIC  CHANGES  211 

the  entire  orchard.  The  conditions  are  comparative  and  furnish  the  hasis 
for  a  fair  comparison  of  the  No.  1418  strain  trees  with  those  of  the  parent 
variety. 

The  yield  of  the  No.  1418  trees  in  the  experimental  plant in<,'  lias  heen 
more  than  double  that  of  the  comparative  trees.  The  No.  1418  fruits  are 
about  twice  the  size  and  weight  of  the  comparative  fruits.  They  are  more 
uniformly  distributed  throughout  the  tree  than  is  the  case  with  the  fruits 
borne  by  the  comparative  trees.  Furthermore,  the  fruits  are  more  uni- 
form in  size,  shape  and  other  characteristics  than  are  the  fruits  of  the 
ordinary  variety.  So  uniform  are  the  No.  1418  fruits  tliat  they  api)ear 
to  have  been  graded  mechanically  as  to  size  as  they  lie  on  tlie  ground 
after  falling. 

The  No.  1418  trees  appear  to  be  more  vigorous  growing  and  develop 
larger  leaves  than  do  the  comparative  trees.  The  leaves  of  the  trees  of 
this  strain  appear  to  be  thicker  and  have  a  tougher  feel  than  do  the  leaves 
of  the  trees  of  the  parent  variety.  In  looking  down  the  rows  one  notices 
that  the  larger  trees  of  the  No.  1418  strain,  with  their  more  luxurious 
and  abundant  foliage,  stand  out  markedly  as  compared  w^ith  the  trees 
and  leaves  of  the  parent  variety. 

The  fruits  of  the  No.  1418  strain  average  about  25-30  to  the  pound  as 
compared  with  an  average  of  from  about  50-60  to  the  pound  as  is  the 
case  of  the  fruits  of  the  parent  variety. 

The  increased  size  of  leaves  and  fruit  and  the  great  vigor  of 
the  tree,  suggest  that  in  this  case  a  "gigas  type"  of  bud- 
mutation  has  occurred  rather  than  change  in  a  single  gene. 
Cytological  study  might  reveal  whether  this  is  the  case  or  not. 
In  plants  with  variegated  leaves,  such  as  Pelargonium 
(Baur)  and  Coleus  (Stout),  it  is  easy  to  change  the  racial 
proportion  of  green  to  white  or  green  to  colored  areas  b}' 
vegetative  selection,  that  is  by  selection  from  among  the 
vegetatively  propagated  offspring  of  a  single  mother  plant. 
Apparently  in  such  cases  what  is  varying  is  the  plastid  con- 
tent of  the  cytoplasm  of  cells  rather  than  their  nuclear 
structure,  but  the  studies  of  Emerson  and  of  Hayes  u])()n 
variegation  of  the  seed-coat  in  maize  show  that  in  this  case 
there  is  a  close  correlation  between  the  somatic  variation 
(seen  in  the  seed-coat)  and  the  variegated  character  trans- 
mitted by  the  embryo  within  the  seed,  so  that  selection  on 
the  basis  of  the  former  is  attended  by  genetic  change  of  ;i 
corresponding  sort  within  the  gametes.  It  cannot  be  doubted 
therefore   that,    in   practically   all   cases   of   variegation    in 


212  GENETICS  AND  EUGENICS 

« 

plants,  real  genetic  changes  are  involved  whenever  selection 
on  the  basis  of  vegetatively  produced  individuals  or  struc- 
tures is  found  to  change  the  racial  character.  Such  a  rela- 
tion has  been  observed  to  hold  in  all  cases  thus  far  carefully 
studied. 

In  regard  to  variegated  seed-coat  in  maize,  Emerson  and 
Hayes  are  agreed  that  the  chief  genetic  changes  occur  in  one 
and  the  same  gene,  which  results  in  producing  a  series  of 
multiple  allelomorphs.  Hayes  recognizes  four  allelomorphs 
in  the  same  series,  Emerson  **at  least  nine  or  ten."  The  num- 
ber is  probably  limited  only  by  the  ability  of  the  observer 
to  discriminate  them.  Besides  variation  in  a  single  gene, 
Hayes  assumes  additional  '* slight  germinal  variations," 
probably  to  be  understood  as  changes  in  other  genetic  loci, 
possibly  located  in  other  chromosomes  and  functioning  as 
'* modifying  factors."  Emerson  finds  that  some  states  of 
the  chief  gene  for  variegated  seed-coat  in  maize  are  appar- 
ently more  stable  than  others,  since  some  members  of  the 
multiple  allelomorph  series  are  observed  to  mutate  less  fre- 
quently than  others.  Thus,  ''self-colored  and  colorless  races 
are,"  he  says,  **as  constant  probably  as  most  Mendelian 
characters,"  but  the  truly  variegated  or  intermediate  types 
mutate  much  more  frequently,  from  one  type  of  variegation 
into  another,  or  even  into  the  more  stable  self-colored  and 
colorless  types. 

Parthenogenesis  in  animals,  like  vegetative  reproduction 
in  plants,  when  as  commonly  it  occurs  without  the  forma- 
tion of  gametes,  affords  an  opportunity  to  observe  how  com- 
mon genetic  changes  are.  For  in  such  cases  no  reduction  of 
the  chromosomes  occurs,  there  is  no  segregation  of  duplicate 
genes,  and  there  is  no  opportunity  for  the  production  of  new 
character  combinations  as  a  result  of  union  of  gametes  in 
fertilization.  Genetic  changes  can  in  such  cases  occur  only 
under  conditions  comparable  with  those  of  bud-variation  in 
plants.  Banta  has  observed  for  long  periods,  extending  into 
hundreds  of  generations,  the  successive  parthenogenetic  gen- 
erations of  small  Crustacea  known  as  water  fleas  (Simoce- 


GENETIC  CHANGES  213 

phalus,  Daphnia)  with  a  view  to  detecting  genetic  changes, 
if  such  occur.  His  attention  has  been  centered  upon  the 
characters  which  distinguish  females  (the  ordinary  partheno- 
genetic  individuals)  from  the  more  rare  males.  He  has  ol)- 
served  the  occurrence  as  mutations  in  Siviocephalus  reiuhis 
and  in  several  different  strains  of  Daphnia  longis-plna,  ul' 
what  are  called  ''sex  intergrades,"  individuals  intermediate 
in  character  between  males  and  females  as  regards  the  sex- 
differentiating  characters  both  primary  and  secondary,  or 
showing  various  combinations  of  the  several  characters 
which  ordinarily  distinguish  the  sexes. ^  That  these  varia- 
tions are  due  to  real  genetic  changes  is  shown  by  their  occur- 
rence in  parthenogenetic  lines  descended  (asexually)  from  a 
common  mother  individual;  that  their  occurrence  is  not  rare 
is  shown  by  the  fact  that  five  out  of  six  lines  of  Daphnia 
under  observation  in  the  year  1918  w^ere  observed  to  give 
rise  to  strains  of  sex  intergrades.  Further,  such  changes  did 
not  occur  in  single  lines  once  only  and  cease  thereafter.  SLx 
lines  were  propagated  from  the  descendants  of  a  single  mu- 
tant sex  intergrade,  and  selected,  three  toward  normal  female- 
ness,  three  toward  maleness.  The  selection  is  characterized 
as  ** somewhat  effective."  "In  most  later  generations,"  says 
Banta,  '*the  stock  in  the  strains  selected  away  from  the 
intergrade  characters  has  been  moderately  or  only  slightly 
intergrade,  while  in  some  cases  the  stock  has  been  almost 
wholly  normal  female.  In  the  strains  selected  to  make  them 
strongly  intergrade,  the  stock  has  usually  been  strongly  in- 
tergrade. ...  In  general  there  is  a  fairly  pronounced  dif- 
ference between  the  characters  of  the  stock  in  strains  selected 
toward  femaleness  and  in  strains  selected  toward  a  more 

1  Banta  enumerates  five  easily  recognized  secondary  sex-characters  in  Daphnia. 
See  Fig.  137.  These  are  (1)  Body  size,  greater  in  females  than  in  males;  (2)  outline 
of  the  head,  forming  a  beak  in  the  female  but  not  in  the  male;  (3)  .fize  ami  character 
of  the  first  antenna,  well  developed  in  males  but  rudimentary  in  females;  (4)  outline 
and  hairiness  of  ventral  anterior  margin  of  carapace,  whic-h  in  males  forms  almost  a 
right  angle  and  is  hairy,  but  in  females  is  rounded  and  hairless;  (5)  character  of 
first  thoracic  appendage,  in  males  with  a  hook-shaped  finger-like  projection,  in  fe- 
males without  hook  and  branched  into  many  long  terminal  filaments. 


214  GENETICS  AND  EUGENICS 

strongly  intergrading  condition."  This  fact  shows  that  minor 
genetic  changes  have  occurred  subsequent  to  the  original 
mutation  either  in  the  same  genetic  locus,  or  loci,  or  in  1 
other  genetic  loci.  Banta  has  shown  that  the  degree  of  in-  ^ 
tergradeness  is  considerably  influenced  by  environmental 
conditions,  but  that  the  facts  are  as  stated,  when  all  needed 
control  observations  are  made.  This  leads  to  the  strong  con- 
viction that  genetic  changes  probably  occur  with  consider- 
able frequency  in  the  parthenogenesis  of  animals  as  well  as 
in  bud- variation  among  plants. ^  There  is,  however,  some 
negative  evidence  on  record.  Ewing  selected  for  forty -four 
parthenogenetic  generations  a  species  of  plant  louse.  Aphis 
avenoB,  which  was  observed  to  vary  as  to  length  of  body, 
length  of  antennae,  and  length  of  cornicles  (honey-dew  tubes) . 
All  the  observed  variations  were  apparently  due  to  environ- 
mental conditions,  because  no  permanent  modification  of 
the  race  was  effected  by  selection.  The  variation  curve  went 
up  and  down  with  change  in  environmental  conditions 
(temperature  and  the  like)  but  returned  to  normal  when 
normal  conditions  were  restored.  Hence  it  appears  that 
genetic  variations  affecting  size  were  not  occurring  with  any 
considerable  frequency,  if  at  all,  in  the  particular  characters 
studied  at  the  particular  time  they  were  studied.  This  is 
not  surprising  when  we  consider  what  a  specialized  organism 
a  plant  louse  is,  adapted  and  limited  to  a  particular  host 
plant.  But  a  single  positive  case,  like  that  studied  by  Banta, 
outweighs  any  number  of  negative  cases  so  far  as  concerns 

^  Banta  (1919)  has  also  studied  the  effects  of  selection,  in  pure  line  cultures  of 
Simocephalus  vetulus,  upon  the  sensitiveness  of  this  species  to  light  stimulation,  as 
measured  by  its  reaction  time.  The  selection  experiment  was  continued  for  54 
months,  181  parthenogenetic  generations.  In  the  first  two-month  period,  no  differ- 
ence could  be  detected  in  the  average  reaction  time  of  plus  selected  and  minus 
selected  strains  of  the  same  pure  line,  but  subsequently  the  two  strains  gradually 
diverged  in  reaction  time  so  that  "in  the  final  ten  generations  the  strain  selected 
for  greater  reactiveness  to  light  had  a  reaction  time  less  than  one-third  as  large  as 
that  for  the  strain  of  the  same  line  selected  for  reduced  reactiveness  to  light."  No 
differences  in  general  vigor  between  the  selected  lines  could  be  detected.  The 
change  was  a  specific  one  in  relation  to  light  reactiveness  and  had  been  attained 
gradually. 


Fig.  137.  Male  tabove)  and  female  (below)  of  Daphina  longispina.  Note  striking  differences  as 
regards  a,  antenna;  br,  breast  (.ventral  anterior  margin  of  carapace);  bk;  beak;  ap,  first  thoracic 
appendage.  Sex  intergrades  may  have  any  combination  or  intermediate  condition  of  the  male  and 
female  characters  shown.     (After  Banta.) 


i  . 

]■  I 


GENETIC  CHANGES  215 

showing  the  possibility  of  the  occurrence  of  genetic  change 
outside  of  sexual  reproduction.^ 

Self-fertilization  among  plants  is  almost  as  favorable  as 
parthenogenesis  or  as  vegetative  reproduction  for  showing 
genetic  changes,  if  they  occur.  For  in  self-fertilization  both 
egg  and  pollen  gametes  are  furnished  by  the  same  parent 
individual.  Johannsen  first  advanced  the  view  that  when 
such  a  parent  individual  is  homozygous  for  all  genetic 
factors,  no  genetic  changes  will  be  observed  among  the  de- 
scendants, which  will  continue  generation  after  generation 
to  constitute  a  '*pure  line."  He  substantiated  this  view  by 
studies  of  size  variation  in  successive  generations  of  self- 
fertilized  beans.  He  found  in  a  number  of  cases  that  no 
change  in  size  resulted  from  selecting  in  successive  genera- 
tions either  the  largest  or  the  smallest  beans  borne  on  the 
same  mother  plant  and  concluded  that  such  plants  were 
homozygous  for  all  genetic  factors  affecting  size  of  seed,  and 
that  the  observed  variations  in  size  upon  which  his  selec- 
tions had  been  based  were  due  to  environmental  agencies 
such  as  the  position  of  the  bean  in  the  pod  and  the  consequent 
amount  of  material  available  for  storage  in  the  seed,  which 
conditions  were  not  subject  to  inheritance. 

The  case  is  very  different  if  one  selects  by  size  beans  borne 
on  a  plant  heterozygous  for  genetic  size  factors  (as  for  ex- 
ample an  Fi  plant  from  a  cross  between  a  large-seeded  and 
a  small-seeded  race  of  beans).  Under  those  conditions  races 
differing  in  average  seed-size  are  quickly  segregated  (Emer- 
son) .  Johannsen's  observations  show  that  genetic  variations 
affecting  the  seed-size  of  beans  are  not  of  frequent  occur- 
rence, yet  he  has  himself  recorded  the  occasional  occurrence 

^  A  very  puzzling  case  of  genetic  change  in  parthenogenesis  is  recorded  by  Na- 
bours.  He  observed  in  grouse  locusts  (Apotettix)  the  development  of  offspring  from 
unfertilized  eggs  which  showed  unmistakable  segregation  of  characters  and  even 
crossing-over  among  linked  characters  for  which  the  mother  was  heterozygous.  All 
the  offspring,  however,  were  of  the  female  sex,  indicating  that  the  eggs  from  which 
they  developed  had  not  undergone  reduction  as  regards  the  sex  determinant,  thougii 
it  would  appear  that  they  must  have  undergone  reduction  as  regards  other  char- 
acters.   Cytological  study  of  such  material  should  prove  interesting. 


216  GENETICS  AND  EUGENICS 

of  mutation  within  a  pure  line  of  beans.  That  such  mutations 
must  occasionally  occur  or  at  least  have  occurred  in  times 
past  is  shown  by  the  very  existence  of  races  differing  in 
genetic  constitution.  By  crossing  these  we  can  produce  in- 
termediates of  any  desired  size.  This  shows  that  the  genetic 
differences  between  them  are  numerous  (on  the  multiple 
factor  h;^^othesis)  and  numerous  genetic  differences  have, 
most  probably,  not  originated  at  one  time  or  place.  Studies 
of  other  self -fertilizing  plants,  such  as  peas,  oats,  wheat  and 
tobacco,  support  the  view  that  genetic  variations  in  such 
species  are  rare  as  compared  with  the  variability  to  be  se- 
cured by  artificially  crossing  different  varieties,  in  which  the 
beneficial  genetic  changes  of  centuries  may  have  accumu- 
lated. All  these  are  immediately  made  available  for  recom- 
bining  in  every  possible  way  with  the  genetic  variations  ac- 
cumulated in  any  other  variety,  when  the  two  are  artificially 
crossed.  Any  advantageous  genetic  variations  which  have 
made  their  appearance  in  a  self-fertilizing  plant,  from  the 
time  it  was  taken  into  cultivation  to  the  present  time,  are 
likely  to  be  found  in  varieties  now  in  cultivation,  since  if 
such  variations  had  survival  value  they  would  naturally  in- 
crease and  come  to  predominate  in  the  crops  of  successive 
years,  even  if  no  conscious  selection  was  exercised.  Acci- 
dental cross-pollinations,  such  as  are  known  to  occur  occa- 
sionally in  any  species  normally  self-fertilized,  would  give 
opportunity  for  combination  to  arise  of  two  or  more  advan- 
tageous genetic  variations,  distinct  in  origin.  Subsequent 
self-fertilization  for  ten  or  more  generations  would  establish 
in  homozygous  lines  all  possible  combinations  of  the  genetic 
factors  introduced  in  the  accidental  cross.  Thus  it  happens 
that  a  field  crop  of  any  seK-fertilizing  plant  contains  a  great 
number  of  pure  lines,  each  considered  by  itself  a  pure-breed- 
ing homozygous  variety.  In  such  cases  the  work  of  the  plant 
breeder  is  very  simple.  He  has  only  to  isolate  the  varieties 
which  nature  gives  him  ready-made  and  test  these  out  to 
determine  which  can  most  profitably  be  grown  in  a  particu- 
lar region  or  under  a  particular  set  of  field  conditions.    In 


GENETIC  CHANGES  217 

farm  practice  elimination  from  the  seed  planted  of  all  but 
the  best  pure  hues,  may  greatly  increase  the  total  yield. 
This  in  many  cases  has  actually  been  done.  The  work  of 
determining  what  are  the  best  pure  lines  and  of  increasing 
these  to  the  exclusion  of  all  others  is  the  main  work  of  the 
plant  breeder.  It  is  a  work  which  will  have  to  be  done  over 
again  generation  after  generation,  because  impurities  will 
creep  in  from  accidental  crossing  with  inferior  ^'a^ieties  or 
from  the  occasional  origin  within  the  pure  line  of  new  genetic 
changes  for  the  worse,  for  quite  as  many  of  this  sort  occur, 
probably,  as  of  those  which  are  for  the  better.  Besides  dis- 
covering and  isolating,  as  pure  lines,  homozygous  strains  of 
favorable  genetic  variations,  as  they  occur  in  commercial 
field  crops,  the  plant  breeder  has  a  second  important  function 
to  perform  in  connection  with  plants  normally  self -fertilized. 
He  may,  by  artificially  crossing  varieties,  combine  the  excel- 
lent qualities  which  they  severally  possess.  It  often  happens 
that  favorable  variations  which  have  arisen  in  the  crops  of 
one  country  are  unknown  in  those  of  another  country.  The 
plant  breeder  may  bring  together  the  best  varieties  of  all 
countries,  determine  the  good  qualities  of  each  and  then  by 
suitable  crosses  combine  these  in  new  varieties  adapted  to 
special  conditions  or  particular  regions.  Never  in  the  history 
of  the  world  has  this  been  done  on  so  extensive  a  scale  or 
with  greater  success  than  in  the  United  States  at  the  present 
time. 

To  return  to  the  point  of  our  departure,  how  common  in 
occurrence  are  genetic  changes  in  self -fertilizing  plants.^  An 
answer  to  this  question  can  be  made  only  in  relative  terms. 
It  is  scarcely  safe  to  assume  that  they  never  occur.  The  very 
existence  of  numerous  genetically  different  pure  lines  in 
every  self -fertilized  crop  shows  that  genetic  changes  have 
occurred  in  the  past,  and  if  so  they  are  doubtless  occurring 
today.  Some  refer  all  such  multiplicity  of  varieties  to  past 
hybridizations  of  species  genetically  different,  but  this  is 
only  referring  to  a  more  remote  period  the  genetic  changes 
which  are  uivolved  in  the  origin  of  the  hypothetical  species 


I 


218  GENETICS  AND  EUGENICS 

themselves.  The  genetic  changes  must  have  occurred  some- 
time if  related  species  really  had  a  common  origin  as  we, 
under  the  Darwinian  theory,  suppose. 

Moreover,  a  cultivated  plant,  regularly  self-fertilized,  the 
sweet  pea,  whose  historic  origin  from  a  single  wild  species  is 
known,  exists  today  in  hundreds  of  true-breeding  varieties 
differing  one  from  another  in  genetic  constitution.  All  these 
genetic  changes  have  occurred  within  a  few  centuries  and  in 
most  if  not  in  all  cases  within  what  were  at  the  time  prob- 
ably *'pure  lines." 

A  common  answer  to  the  question  proposed  is  that  genetic 
changes  in  pure  lines  are  comparatively  rare.  Rare  in  com- 
parison with  what.^  With  the  genetic  variations  already 
existing  in  the  same  species.  But  the  latter  are  accumula- 
tions of  the  genetic  changes  of  centuries,  or  in  the  case  of 
cultivated  wheat,  of  thousands  of  years.  Is  it  surprising  that 
in  comparison  with  such  accumulations  of  variations,  the 
variations  observed  contemporaneously  to  occur  in  pure 
lines  are  relatively  few?  Practically,  it  would  be,  as  has 
often  been  said,  a  '* waste  of  time"  to  look  for  the  occurrence 
of  favorable  genetic  changes  within  pure  lines  of  self -fertiliz- 
ing plants,  so  long  as  a  wealth  of  untested  varieties  exists 
ready  made  in  every  commercial  variety  of  such  crop,  and 
an  even  greater  number  of  new  varieties  may  be  created  by 
crossing  the  best  existing  varieties.  But  this  is  not  to  be 
regarded  as  evidence  that  genetic  changes  have  not  come 
about  in  the  past  exactly  as  they  are  coming  about  today, 
within  lines  pure  or  otherwise. 


CHAPTER  XXV 

GENETIC  CHANGES  IN  BISEXUAL  REPRODUCTION 

How  common  are  genetic  changes  in  ordinary  bisexual  re- 
production? This  question  also  can  be  answered  only  in 
relative  terms.  Few  organisms  have  been  studied  intensively 
enough  and  for  enough  generations  in  succession  to  enable 
us  to  answer  the  question  intelligently.  Drosophila  melano- 
gaster  has  probably  been  studied  more  thoroughly  than  any 
other  species,  these  studies,  because  of  the  rapid  reproduc- 
tion of  Drosophila,  extending  over  hundreds  of  successive 
generations.  No  other  organism  has  yielded  such  a  great 
number  of  known  distinct  genetic  variations,  but  at  first 
their  discovery  came  rather  slowly.  Improved  technique  and 
training  on  the  part  of  observers  enabled  them  to  recognize 
more  and  more  genetic  changes.  Those  discovered  within 
ten  years  have  mounted  in  number  into  the  hundreds.  There 
is  reason  to  think  that  a  goodly  proportion  of  these  genetic 
changes  have  actually  occurred  (not  merely  been  discovered) 
during  the  period  of  laboratory  study  of  Drosophila  at  Co- 
lumbia University.  Some  of  them  have  been  observed  to 
occur  independently  at  different  periods  and  in  unrelated 
stocks  of  flies.  This  indicates  that  in  the  best-known  genet- 
ically of  all  organisms  the  genes  are  extremely  numerous  and 
are  subject  to  rather  frequent  changes,  for  we  are  acquainted 
only  with  such  genes  as  have  revealed  themselves  by  under- 
going change.  The  first  discovered  gene  was  that  for  white 
eye.  In  all  eight  different  allelomorphic  forms  of  this  gene 
have  now  been  described,  viz.,  (1)  white,  (2)  tinged,  (3)  buff, 
(4)  eosin,  (5)  cherry,  (6)  blood,  (7)  coral,  and  (8)  red.  They 
form  a  series  of  grades  of  increasing  intensity  of  red  pigmen- 
tation, each  one  having  made  its  appearance  independently 
of  the  others.  Bridges  has  made  an  intensive  study  of  juinor 
genetic  variations  in  one  of  these  seven  grades,  viz.,  c\)sin, 

219 


220  GENETICS  AND  EUGENICS 

the  middle  one  of  the  series.  He  finds  that  in  a  pure  culture 
of  eosin,  the  intensity  of  the  pigmentation  may  vary  from  a 
"deep  pink  darker  than  eosin"  to  a  **pure  white,"  through 
the  modifying  action  of  eight  other  factors,  '*in  origin  en- 
tirely independent  of  one  another"  and  located  each  at  a 
different  genetic  locus,  four  being  in  chromosome  II  and  one 
in  chromosome  III,  the  others  not  having  been  definitely 
located.  Seven  of  the  eight  modifying  factors  act  as  diluters 
or  lighteners  of  unmodified  eosin,  one  only  acting  as  a  dark- 
ener.  They  are  in  the  order  of  their  darkening  (or  lightening) 
effects,  (1)  dark,  (2)  pinkish,  (3)  cream  c,  (4)  cream  b, 
(5)  cream  a,  (6)  cream  III,  (7)  cream  II,  and  (8)  whiting. 
**Each  of  these  genes  arose  by  mutation,"  while  the  stocks 
were  under  continuous  study,  ''by  the  transformation  of  the 
materials  of  a  particular  locus  into  a  new  form  having  a 
different  effect  upon  the  developmental  processes."  The 
eye-color  mutations  observed  to  occur  in  Drosophila  since 
Morgan's  discovery  of  white  eye  are  so  numerous  that 
Bridges  classifies  them  in  per  cents,  as  60  per  cent  general  or 
non-specific  modifiers  of  eosin,  such  as  vermilion  and  pink, 
22  per  cent  specific  modifiers  of  eosin,  and  18  per  cent  allelo- 
morphs of  eosin.  He  continues,  **It  is  probable  that  muta- 
tion (change  in  single  genes)  is  very  much  more  frequent 
than  appears,  since  a  great  many  mutations  are  of  very 
slight  somatic  effect  and  would  pass  undetected  except  that 
certain  characters  such  as  eosin  eye-color,  truncate  wings, 
beaded  wings,  and  a  few  others,  are  peculiarly  sensitive  dif- 
ferentiators for  eye-color  and  wing-shape  genes,  etc."  Here 
we  have  a  picture  of  genetic  mutability  in  the  most  carefully 
studied  of  organisms,  occurring  contemporaneously,  which 
affords  all  the  material  needed  for  selection  either  natural  or 
artificial  to  act  upon  in  either  darkening  or  lightening  the 
eye-color  by  a  series  of  progressive  steps,  if  such  change 
should  be  found  advantageous  or  desirable. 

Among  organisms  reproducing  sexually,  the  evening- 
primrose  has  probably  been  studied  more  intensively  than 
any  other  except  Drosophila.   But  it  is  impossible  to  say  in 


BISEXUAL  REPRODUCTION  221 

the  case  of  Oenothera  to  what  extent  variation  in  single 
genes  is  occurring,  because  those  who,  following  De  Vries, 
have  studied  ^'mutation"  in  this  multifarious  genus  of 
plants,  have  directed  their  attention,  almost  without  excep- 
tion, to  the  major  variations  which  they  have  called  *'nui- 
tations,"  and  have  neglected  or  denied  the  existence  of 
minor  genetic  variations,  such  as  have  been  studied  in  much 
detail  in  Drosophila.  In  some  cases,  such  as  the  *'gigiis"  and 
the  ''lata"  types  of  mutation,  irregularities  of  cell-division 
seem  to  have  resulted  in  duplication  of  entire  chromosomes, 
or  of  the  entire  set  of  normal  chromosomes.  As  a  comple- 
mentary phenomenon  we  should  expect  entire  chromosomes 
to  be  lost  from  the  germ-cell  in  other  cases,  and  this  may 
possibly  be  the  explanation  of  some  classes  of  Oenothera 
mutants  whose  associated  cytological  conditions  have  not 
been  determined.  In  the  presence  of  such  striking  genetic 
changes  it  is  not  surprising  that  variations  in  single  genes 
have  scarcely  been  detected,  although  the  '*nanella"  and 
*'rubri-calyx"  mutants  may  be  mentioned  as  manifesting 
simple  uni-factorial  Mendelian  inheritance.  It  seems  prob- 
able that  when  the  minor  variations  of  Oenothera  are  studied 
as  intensively  as  its  peculiar  "mutations"  have  been  studied, 
they  will  be  found  to  be  not  less  frequent  in  occurrence. 

In  domesticated  mammals  and  birds,  where  asexual  repro- 
duction, self-fertilization,  and  parthenogenesis  are  unknown, 
and  where  so  much  racial  or  family  hybridization  is  con- 
stantly being  carried  on  with  a  view  to  increasing  vigor  or 
variability,  it  is  difficult  to  say  how  much  of  the  genetic  vari- 
ability is  of  contemporaneous  origin  and  how  much  of  it  hiis 
been  handed  down  in  the  stock  from  previous  generations. 
Theoretically,  it  should  be  possible  to  make  any  stock  of 
animals  homozygous  for  practically  every  gene  by  inbreed- 
ing continued  for  twenty  or  more  generations,  mating  brother  { 
with  sister,  parent  with  offspring,  cousin  with  cousin,  or 
uncle  with  niece  (Jennings) .  If  this  is  done  and  genetic  varia- 
tion is  subsequently  observed  to  occur,  this  must  have  orig- 
inated after  the  stock  had  been  purified.  By  this  means  we 


222  GENETICS  AND  EUGENICS 

get  an  idea  of  how  frequently  genetic  changes  are  happening. 
Such  purification  of  stock  has  rarely  been  undertaken.  Miss 
King  has  inbred  rats,  brother  with  sister,  for  25  generations 
and  the  resulting  stock  has  been  studied  as  to  genetic  char- 
acter chiefly  in  respect  to  sex  ratio.  In  the  course  of  the 
inbreeding,  selection  was  made  in  two  different  lines  for 
opposite  changes  in  the  sex  ratio;  in  Series  A  for  a  high  ratio 
of  males  to  females,  and  in  Series  B  for  a  low  ratio  of  males 
to  females.  The  result  in  6,274  young  of  the  25  generations 
of  the  A  series  was  a  ratio  of  122.3  males  to  100  females.  In 
5,893  young  of  the  B  series,  the  ratio  was  81.8  males  to  100 
females.  These  very  different  results  were  secured  within 
the  first  10  or  12  generations  of  selection,  after  which  progress 
in  the  direction  of  the  selection  was  negligible.  This  indicates 
that  the  genetic  factors  responsible  for  the  changes  were 
already  in  existence  in  the  stock  at  the  beginning  of  the 
selection  and  were  gradually  sorted  out  and  rendered  ho- 
mozygous in  the  early  part  of  the  selection  period,  and  that 
new  genetic  changes  appreciable  in  amount  did  not  appear 
subsequently. 

Selection  was  made  simultaneously  for  large  size  in  the 
course  of  the  inbreeding  experiments  of  Miss  King  and  this 
resulted  in  producing  inbred  races  which  were  larger  than 
the  unselected  stocks  from  which  they  were  derived.  The 
maximum  size  was  attained  as  early  as  the  seventh  inbred 
generation,  possibly  earlier,  as  the  seventh  generation  is  the 
earliest  one  for  which  comparable  data  are  available.  The 
inbred  races  maintained  throughout  the  entire  period,  up  to 
the  twenty-fifth  inbred  generation,  their  superiority  in  size 
over  the  control  stocks  from  which  the  inbred  strains  had 
originated,  but  no  evidence  was  found  that  genetic  size  factors 
had  changed  in  the  period  between  the  seventh  and  twenty- 
fifth  inbred  generations.  Observations  were  made  also  on 
fecundity  as  indicated  by  size  of  litter  in  the  inbred  rats. 
No  change  was  observed  in  this  character  as  a  result  of  the 
inbreeding.  The  average  size  of  litter  was  for  the  inbred 
series  7.5  young,  for  the  stock  albinos  used  as  a  control,  6.7 


«^«^«>^ 


12 


15 


18 

Fig.  138.  Grades  1-18  of  white  spotting  as  seen  in  Dutch  rabbits.  By  systematic  selection  the  average 
grade  of  a  race  of  Dutch  rabbits  may  be  gradually  but  peruiamntly  changi-d  either  in  a  plus  or  in  a 
minus  direction.    Dutch  spotting  is  allelomorphic  with  English  (Fig.  liS). 


I 


II 


Fig.  139.  TjTiical  examples  of  three  races  of  Dutch  rabbits,  each  having  a  different  and  characteristic 
amount  or  distribution  of  white  spotting.  The  top  figure  represents  the  "  while  "  rao-;  niiddlc  figure, 
the  "tan"  race;  lowest  figure,  the  "dark"  race.  Each  is  allelomorphic  with  the  others  in  crosses  but 
segregates  in  a  slightly  modified  form. 


No 
50- 


40- 


30- 


20- 


10- 


0 
10- 


\~VI, 


0' 


F,,  DxW 


\\  \J     ^      M  \    ■     << 


Grade,    0      I 


16    17 


Fig.  140.  Graphic  presentation  of  the  variation  in  grade  of  two  races  of  Dutch  rabbits,  "dark  (D) 
and  "white"  (W),  and  results  of  intercrossing  them.  At  the  top  is  shown  the  gra.  e  distribution  of 
each  uncrossed  race;  below  is  shown  the  grade  distribution  of  Fi  animals,  of  F2  amma  s.  an.l  of  anmiaLs 
produced  by  back-crosses  of  Fi  with  each  parent  race.  Note  that  the  extracted  D  or  AN  groups  diverge 
less  from  each  other  than  the  uncrossed  D  and  W  groups. 


II 


BISEXUAL  REPRODUCTION  223 

young  per  litter.  At  the  beginning  of  the  inbreeding  experi- 
ment strains  of  large,  vigorous,  rap  id -growing  and  fecund 
animals  were  isolated  from  the  general  stock,  and  those 
characters  seem  to  have  been  maintained  w^ithout  diminu- 
tion under  the  continuous  selection  exercised  in  choosing  as 
breeders  the  largest  and  best  nourished  individuals  from 
each  litter,  but  no  evidence  is  forthcoming  of  further  pro- 
gressive genetic  changes. 

In  hooded  rats  inbred,  but  not  exclusively  in  brother- 
sister  ma  tings,  for  twenty  generations,  selection  has  been 
made  successfully  for  change  of  the  hooded  pattern  in  op- 
posite directions,  to  make  the  race  as  white  as  possible  in  one 
line,  and  as  dark  as  possible  in  another  line.  (See  Tables  27 
and  28.)  Genetic  variability  decreased  somewhat  during 
the  first  seven  or  eight  generations,  which  probably  suflBced 
to  eliminate  most  of  the  genetic  variability  originally  present 
in  the  stock  as  modifying  factors.  But  subsequently  the 
variability  as  measured  by  the  standard  deviation  showed 
little  change  up  to  the  end  of  the  experiment  in  generation 
21  when  the  selected  races  died  out  owing  to  the  prevalence 
of  disease  and  infertility.  The  case  seems  to  be  best  inter- 
preted as  one  in  which  minor  genetic  changes  are  continually 
occurring,  so  that  selection  utilizing  these  may  move  the 
racial  mode  and  mean  either  in  a  plus  or  in  a  minus  direction 
without  encountering  impassable  limits  short  of  an  all  white 
or  an  all  black  condition.  There  is  a  strong  parallelism  be- 
tween the  variability  of  the  white-spotting  pattern  in  rats 
and  other  mammals  and  the  variability  of  variegated  seed- 
coat  in  maize  and  of  variegated  foliage  in  a  great  many 
species  of  plants.  In  both  sets  of  cases  an  unstable  mosaic 
of  alternative  characters  exists,  pigmentation  and  nonpig- 
mentation;  somatic  variation  in  the  relative  proportions  of 
the  balanced  characters  is  constantly  occurring,  and  germi- 
nal variation  of  a  similar  nature  very  connnonly  occurs  at 
the  same  time  as  the  somatic  Variation,  so  that  selection  on 
the  basis  of  the  somatic  variation  effects  germinal  change  in 
the  race.    The  variabihty  (or  ''mutability")  in  the  case  of 


224  GENETICS  AND  EUGENICS 

plants  with  variegated  seed-coat  or  foliage  extends  into  end 
stages  of  the  series  which  are  wholly  colored  or  wholly  color- 
less, which  stages  seem  to  be  more  stable  than  the  inter- 
mediate (mosaic)  stages,  as  pointed  out  by  Emerson.  It  is 
to  be  regretted  that  in  the  selection  experiments  with  rats 
similar  end  stages  were  not  reached  before  the  selected  races 
perished.  In  the  case  of  Dutch  rabbits  (Figs.  138-140)  the  all- 
white  condition  has  been  recorded  once,  and  the  all-colored 
condition  is  often  found  in  animals  known  to  be  either 
heterozygous  or  homozygous  for  some  form  of  white  spotting. 

Two  different  explanations  of  cases  of  this  class  in  animals 
and  plants  have  been  suggested.  (1)  On  one  view  the  chief 
genetic  locus  mutates  frequently  producing  multiple  alle- 
lomorphs more  or  less  stable  (Emerson),  but  these  multiple 
allelomorphs  may  be  supplemented  in  action  by  minor  modi- 
fying genes  (Hayes).  (2)  On  another  view  the  chief  gene  is 
as  stable  as  other  genes  and  the  ordinary  genetic  variability 
is  due  exclusively  to  modifying  genes  (MacDowell,  Pearl, 
Sturtevant) .  If  the  chief  gene  is  really  less  stable  in  the  case 
of  these  mosaic  characters  than  in  ordinary  cases,  as  the 
descriptive  term  used  by  DeVries,  **ever  sporting  char- 
acters," would  seem  to  imply,  at  least  in  the  case  of  plants, 
it  may  be  because  a  mosaic  condition  exists  at  the  genetic 
locus  itself.  In  variegated  plants  the  character  of  the  mosaic 
in  particular  parts  of  the  plant  corresponds  roughly  with 
the  character  of  variegation  transmitted  by  flowers  arising 
in  those  same  parts  of  the  plant  whether  egg-cells  or  pollen- 
cells  are  the  vehicles  of  transmission,  which  suggests  actual 
variation  in  the  genetic  locus  involved  rather  than  change  in 
modifying  genes.     (See  Figs.  135,  136.) 

MacDowell  inbred,  brother  with  sister,  a  race  of  Droso- 
phila  possessing  a  recessive  Mendelian  character,  extra 
bristles,  for  49  generations,  selecting  meanwhile  in  different 
lines  for  high  and  for  low  number  of  extra  bristles.  For  about 
eight  generations  the  selection  was  effective  after  which  no 
material  change  was  observed  attributable  to  the  selection. 
MacDowell  concludes  that  at  the  beginning  of  the  experi- 


BISEXUAL  REPRODUCTION  225 

ment  a  number  of  genetic  factors  modifying  l)ristle  number 
were  present  in  the  stock  in  heterozygous  condition.  Selec- 
tion attending  the  inbreeding  served  to  ehminate  certain  of 
these  from  one  race  and  to  estabhsh  them  in  homozygous 
condition  in  the  other  race,  after  which  no  genetic  changes 
would  be  observed  unless  they  arose  de  novo.  As  MacDow  ell 
was  unable  to  detect  any  such  changes,  he  concludes  that 
none  were  occurring.  Payne  has  carried  out  a  similar  selec- 
tion experiment  for  changed  number  of  bristles,  in  another 
race  of  Drosophila,  starting  with  the  descendants  of  a 
single  **mutant"  individual  with  "reduced"  bristle  number, 
which  appeared  in  an  "extra-bristle"  strain.  Selection  was 
made  among  the  descendants  of  a  single  pair  of  flies  and  was 
carried  in  brother-sister  matings  in  a  minus  selected  line  for 
64  generations,  and  in  a  plus  selected  line  for  60  generations. 
Toward  the  end  of  the  experiment,  the  flies  of  the  minus  line, 
in  from  96  to  100  per  cent  of  all  cases,  were  without  bristles. 
This  degree  of  purity  was  attained  gradually  during  the  first 
seventeen  generations  of  minus  selection,  after  which  no 
further  genetic  change  was  observed.  But  in  the  plus  selec- 
tion, toward  the  normal  number  of  bristles,  four,  in  other 
races  of  Drosophila,  progress  continued  longer,  reaching  its 
maximum  in  the  55th  generation  when  64  per  cent  of  the 
individuals  possessed  four  bristles.  The  two  selected  lines, 
plus  and  minus,  had  thus  become  very  different  as  a  result  of 
selection.  Payne  finds  evidence  that  two  or  possibly  three 
genetic  factors  affecting  bristle  number  were  present  in  the 
plus  line,  two  of  them  being  sex-linked,  but  that  in  the 
minus  line  only  a  single  factor  was  present.  (Figs.  141, 14''2.) 
Zeleny  inbred  a  race  of  Drosophila  possessing  a  dominant 
character,  bar  eye,  meanwhile  selecting  for  high  and  for  low 
grades  of  the  character  (number  of  ommatidia).  During  the 
course  of  the  selection  two  striking  mutations  were  observed 
of  the  gene  under  study,  one  a  reverse  mutation  to  "full" 
(normal)  eye,  the  other  a  mutation  in  the  direction  of  selec- 
tion toward  a  more  reduced  condition  of  the  eye,  and  called 
*'ultrabar."    The  average  size  of  the  eye  in  these  thrw  alle- 


£26  GENETICS  AND  EUGENICS 

lomorphic  states  of  the  *'bar"  gene  is  as  follows:  Full 
(normal)  eye,  849,.8  facets;  bar  eye,  75.6  facets;  ultra-bar 
eye,  23  facets.  Zeleny  also  observed  lesser  mutations  of  the 
gene  for  bar,  which  made  their  appearance  during  42  care- 
fully controlled  generations  of  selection  for  low  and  for  high 
facet  number  in  brother-sister  matings.  Aside  from  muta- 
tions in  the  gene  for  bar,  Zeleny  observed  genetic  differences 
between  his  high  selected  and  low  selected  lines  which  he 
ascribes  to  "accessory  factors  outside  of  the  sex  chromosome 
in  which  the  bar  gene  is  located."  These  when  present  in 
heterozygous  state  are  speedily  sorted  out  by  selection, 
which  '* ceases  to  be  effective  after  three  to  five  generations." 
*' There  is,  however,  no  limit  to  the  possibilities  of  selection 
if  the  occasional  mutants  are  included  in  the  series,  and  two 
at  least  of  these,  reversal  to  full  and  ultra-bar,  have  been 
shown  to  be  changes  in  the  bar  gene  itself." 

We  may  conclude  that  the  amount  of  genetic  change 
which  is  occurring  at  the  present  time  is  greater  as  regards 
some  characters  than  as  regards  others,  and  is  probably 
greater  in  some  organisms  than  in  others.  But  in  any  group 
of  organisms  capable  of  interbreeding,  which  has  been  divided 
for  any  length  of  time  into  non-interbreeding  groups  (races, 
breeds,  or  strains)  genetic  differences  of  one  sort  or  another 
will  probably  be  found  to  have  arisen,  when  an  intensive 
study  of  the  matter  is  made.  If  so,  we  must  conclude  that 
genetic  changes  are  probably  occurring  with  appreciable 
frequency  in  most,  if  not  in  all  organisms.  But  it  should  be 
stated  emphatically  that  the  amount  of  variability  to  be 
detected  by  selection  within  pure  lines  is  in  all  cases  small 
as  compared  with  that  which  can  be  secured  by  crossing 
different  strains,  breeds  or  varieties,  which  have  long  been 
established  within  the  species.  For  in  pure  line  selection  only 
genetic  changes  occurring  during  the  process  of  selection  are 
likely  to  be  revealed,  but  following  a  variety  cross,  all  pos- 
sible recombinations  may  be  expected  of  the  genetic  changes 
which  have  occurred  since  the  two  parent  groups  diverged 
from  each  other. 


K^—r-. -, ,1 


gen«iationt 


Fig.  141.  Results  of  selection  for  a  reduced  number  of  bristles  continued  for  64  generations  among 
the  inbred  descendants  of  a  single  pair  of  flies.  The  heavy  line  shows  the  percentage  of  flies  with- 
out bristles  in  each  generation.  Note  that  little  change  occurs  after  the  17th  inbred  generation. 
(After  Payne.) 


I         1»'       12        M        I* 

8f ne  ra  1 1 Da« 


■nc- 


FiG.  142.  Results  of  selection  for  an  increased  number  of  bristles  made  throughout  60  inbred  g*-- 
rations  upon  the  same  initial  stock  as  is  mentioned  in  the  description  of  Fig.  141.  The  hea\->'  line 
shows  the  percentage  of  four-bristled  (normal)  flies  in  each  generation.  Note  the  progressiw 
increase  which  continued  as  late  as  generation  55.     (After  Payne.) 


CHAPTER  XXVI 

INBREEDING  AND  CROSSBREEDING 

It  is  the  opinion  of  most  experienced  animal  breeders  that 
close  inbreeding  should  be  avoided  because  it  has  a  tendency 
to  decrease  the  size,  vigor  and  fecundity  of  the  race  in  which 
it  is  practiced.  Many  even  believe  that  it  leads  to  the  pro- 
duction of  abnormal  individuals  or  monstrosities.  On  the 
other  hand  some  of  those  who  have  had  greatest  success  in 
producing  new  or  improved  breeds  of  domesticated  animals 
have  practiced  the  closest  kind  of  inbreeding  and  attribute 
their  success  in  part  to  this  fact. 

In  human  society  we  find  a  nearly  unanimous  condemna- 
tion of  the  marriage  of  near-of-kin.  Nearly  all  peoples, 
civilized  or  uncivilized,  forbid  it.  Only  exceptionally,  as  in 
the  case  of  the  royal  families  of  ancient  Egypt  and  ancient 
Peru,  has  the  marriage  of  brother  and  sister  been  sanctioned. 
The  underlying  reason  in  such  cases  was  a  belief  that  the 
family  in  question  constituted  a  superior  race  whose  members 
could  find  no  fit  mates  outside  their  own  number.  There  was 
probably  no  thought  that  inbreeding  itself  was  beneficial  l)iit 
only  a  desire  to  conserve  the  superior  excellence  believed  to 
reside  in  certain  individuals.  The  same  considerations,  j^rob- 
ably  have  led  to  the  occasional  practice  of  inbreeding  in 
animal  husbandry,  viz.,  the  desire  to  conserve  and  per]:)etu- 
ate  the  superiority  of  particular  individuals. 

If  we  inquire  into  the  biological  foundation  of  the  idea  that 
inbreeding  is  harmful,  we  come  upon  seemingly  conflicting 
evidence.  No  generalization  can  be  drawn  which  is  applic- 
able to  all  organisms. 

By  inbreeding  we  mean  the  mating  of  closely  relatcnl  in- 
dividuals. As  there  are  different  degrees  of  relationshi])  be- 
tween individuals,  so  there  are  different  degrees  of  inbreeding. 
The  closest  possible  inbreeding  occurs  among  plants  in  what 

227 


228  GENETICS  AND  EUGENICS 

we  call  self-pollination,  in  which  the  egg-cells  of  the  plant  are 
fertilized  by  pollen-cells  produced  by  the  same  individual.  A 
similar  phenomenon  occurs  among  some  of  the  lower  animals, 
notably  among  parasites.  But  in  all  the  higher  animals, 
including  the  domesticated  ones,  such  a  thing  is  impossible 
because  of  the  separateness  of  the  sexes.  For  here  no  indi- 
vidual produces  both  eggs  and  sperm.  The  nearest  possible 
approach  to  self-pollination  is  in  such  cases  the  mating  of 
brother  with  sister,  or  of  parent  with  child.  But  this  is  less 
close  inbreeding  than  occurs  in  self-pollination,  for  the 
individuals  mated  are  not  in  this  case  identical  zygotes, 
though  they  may  be  similar  ones. 

It  has  long  been  known  that  in  many  plants  self-pollina- 
tion is  habitual  and  is  attended  by  no  recognizable  ill  effects. 
This  fortunate  circumstance  allowed  Mendel  to  make  his 
remarkable  discovery  by  studies  of  garden  peas,  in  which  the 
flower  is  regularly  self -fertilized,  and  never  opens  at  all  unless 
made  to  do  so  by  some  outside  agency.  Self-pollination  is 
also  the  rule  in  wheat,  oats,  and  the  majority  of  the  other 
cereal  crops,  the  most  important  economically  of  cultivated 
plants.  Crossing  can  in  such  plants  be  brought  about  only 
by  a  difficult  technical  process,  so  completely  adapted  is  the 
plant  to  self-pollination.  And  crossing,  too,  in  such  plants  is 
of  no  particular  benefit,  unless  by  it  one  desires  to  secure 
new  combinations  of  unit-characters. 

In  maize,  or  Indian  corn,  however,  among  the  cereals,  the 
case  is  quite  different.  Here  enforced  self-pollination  results 
in  small  unproductive  plants,  lacking  in  vigor.  But  racial 
vigor  is  fully  restored  by  a  cross  between  two  depauperate, 
unproductive  individuals  obtained  by  self-fertilization,  as  has 
been  shown  by  Shull.  This  result  is  entirely  in  harmony 
with  those  obtained  by  Darwin,  who  showed  by  long-con- 
tinued and  elaborate  experiments  that  while  some  plants  do 
not  habitually  cross  and  are  not  even  benefited  by  crossing, 
yet  in  many  other  plants  crossing  results  in  more  vigorous 
and  more  productive  offspring;  that  further,  the  advantage 
of  crossing  in  such  cases  has  resulted  in  the  evolution  in 


INBREEDING  AND  CROSSBREEDING  2!29 

many  plants  of  floral  structures,  which  insure  crossing 
through  the  agency  of  insects  or  of  the  wind. 

In  animals  the  facts  as  regards  close  fertilization  are  similar 
to  those  just  described  for  plants.  Some  animals  seem  to  be 
indifferent  to  close  breeding,  others  will  not  tolerate  it.  Some 
hermaphroditic  animals  (those  which  produce  both  eggs  and 
sperm)  are  regularly  seK-fertilized.  Such  is  the  case,  for 
example,  with  many  parasitic  flatworms.  In  other  cases 
seK-fertilization  is  disadvantageous.  One  such  case  I  was 
able  to  point  out  some  twenty  years  ago,  in  the  case  of  a  sea- 
squirt  or  tunicate,  Ciona.  The  same  individual  of  Ciona 
produces  and  discharges  simultaneously  both  eggs  and  sperm, 
yet  the  eggs  are  rarely  seK-fertilized,  for  if  self-fertilization  is 
enforced  by  isolation  of  an  individual,  or  if  seK-fertilization  is 
brought  about  artificially  by  removing  the  eggs  and  sperm 
from  the  body  of  the  parent  and  mixing  them  in  sea  water, 
very  few  of  the  eggs  develop,  —  less  than  10  per  cent.  But  if 
the  eggs  of  one  individual  be  mingled  with  the  sperm  of  any 
other  individual  whatever,  practically  all  of  the  eggs  are 
fertilized  and  develop. 

In  plants  much  attention  has  been  given  to  the  problem  of 
self -sterility  by  East,  Stout,  Dorsey,  and  others.  The  case 
of  native  American  plums  is  as  simple  as  any.  All  varieties 
investigated  by  Dorsey  were  found  to  be  self-sterile.  If  self- 
pollinated,  they  set  no  fruit,  either  because  the  pollen  grains 
fail  to  germinate  or  because  the  pollen  tubes,  if  formed,  grow 
too  slowly  to  reach  and  fertilize  the  ovules.  Not  only  are 
all  varieties  self -sterile,  some  are  also  cross-sterile,  i.  e.,  sterile 
when  crossed  with  each  other.  It  is  probable  that  such 
varieties  have  inherited  a  similar  genetic  constitution,  so 
that  the  pollen  of  one  reacts  toward  the  pistil  of  another  as 
toward  pistils  of  its  own  plant.  In  support  of  this  view  it 
may  be  said  that  East  and  Park  found  that  the  F.  phmts 
produced  by  crossing  Nicotiana  Forgeiiana  with  N.  data  fell 
into  four  groups,  all  the  plants  in  each  group  being  mutually 
cross-sterile  but  fertile  with  any  plant  of  the  other  three 
groups.    The  obvious  conclusion  is  that  the  plants  of  each 


230  GENETICS  AND  EUGENICS 

group  were  similar  in  constitution  as  regards  factors  affect- 
ing fertility,  and  that  some  dissimilarity  is  necessary  to 
enable  the  pollen  of  one  individual  or  variety  to  grow  vigor- 
ously on  the  stigma  of  another  individual  or  variety.  The 
phenomenon  of  self-sterility  accordingly  involves  the  prin- 
ciple of  heterosis.    (See  Chapter  XXVII.) 

In  the  great  majority  of  animals,  as  in  many  plants,  seK- 
fertilization  is  rendered  wholly  impossible  by  separation  of 
the  sexes.  The  same  individual  does  not  produce  both  eggs 
and  sperm,  but  only  one  sort  of  sexual  product.  But  among 
sexually  separate  animals  the  same  degree  of  inbreeding 
varies  in  its  effects.  The  closest  degree,  mating  of  brother 
with  sister,  has  in  some  cases  no  observable  ill  effects.  Thus, 
in  the  case  of  a  small  fly,  Drosophila,  my  pupils  and  I  bred 
brother  with  sister  for  fifty-nine  generations  in  succession 
without  obtaining  a  diminution  in  either  the  vigor  or  the 
fecundity  of  the  race,  which  could  with  certainty  be  attrib- 
uted to  that  cause.  A  slight  diminution  was  observed  in 
some  cases,  but  this  was  wholly  obviated  when  parents  were 
chosen  from  the  more  vigorous  broods  in  each  generation. 
Nevertheless  crossing  of  two  inbred  strains  of  Drosophila, 
both  of  which  were  doing  well  under  inbreeding,  produced  off- 
spring superior  in  productiveness  to  either  inbred  strain. 
Even  in  this  case,  therefore,  though  inbreeding  is  tolerated, 
crossbreeding  has  advantages. 

In  the  case  of  many  domesticated  animals,  it  is  the  opin- 
ion of  experienced  breeders,  supported  by  such  scientific 
observations  as  we  possess,  that  decidedly  bad  effects  follow 
continuous  inbreeding.  Bos  (1894)  practiced  continuous  in- 
breeding with  a  family  of  rats  for  six  years.  No  ill  effects 
were  observed  during  the  first  half  of  the  experiment,  but 
after  that  a  rapid  decline  occurred  in  the  vigor  and  fertility 
of  the  race.  The  average  size  of  litter  in  the  first  half  of  the 
experiment  was  about  7.5,  but  in  the  last  year  of  the  experi- 
ment it  had  fallen  to  3.2,  and  many  pairs  were  found  to  be 
completely  sterile.  Diminution  in  size  of  body  also  attended 
the  inbreeding,  amounting  to  between  8  and  20  per  cent. 


1005^ 


8 


10 


Segregating  Cenerationa 

Fig.  143.  Graphs  showing  the  progressive  reduction  of  heterozygous  individuals  in  a  p«ipuIation  of 
self-fertilized  plants,  starting  with  all  individuals  heterozygoles.  Four  east-s  are  shown,  in  which  the 
number  of  independent  allelomorphs  is  respectively  1,  5,  10,  or  15.     (After  East  and  Jones.) 


INBREEDING  AND  CROSSBREEDING  231 

Experiments  made  by  Weismann  confirm  those  of  Bos  as 
regards  the  falHng  off  in  fertility  due  to  inl)r(M'(h*ng.  Vov 
eight  years  Weismann  bred  a  colony  of  mice  started  from 
nine  individuals,  —  six  females  and  three  males.  'Jlie  experi- 
ment covered  twenty-nine  generations.  In  tlie  first  ten  gen- 
erations the  average  number  of  young  to  a  litter  was  0.1; 
in  the  next  ten  generations,  it  was  5.6;  and  in  the  last  nine 
generations,  it  had  fallen  to  4.2. 

But  recent  inbreeding  experiments  with  rats  carried  on  at 
the  Wistar  Institute  by  Dr.  Helen  King  give  results  quite  at 
variance  with  those  of  Bos  and  Weismann.  She  finds,  as 
was  found  to  be  the  case  in  Drosophila,  that  races  of  large 
size  and  vigor  and  of  complete  fertility  may  be  maintained 
under  the  closest  inbreeding,  if  the  more  vigorous  individuals 
are  selected  as  parents.  By  this  means  she  seems  to  have 
secured  races  of  rats  which  are  relatively  immune  to  injurious 
effects  from  inbreeding.  My  own  experience  with  rats  inbred 
within  lines  of  narrow  selection  for  seventeen  generations  is 
that  races  of  fair  vigor  and  fecundity  can  be  maintained 
under  these  conditions,  but  that  when  two  of  these  inbred 
races  are  crossed  with  each  other,  even  though  they  had  their 
origin  in  a  small  common  stock  many  generations  earlier,  an 
immediate  and  striking  increase  of  vigor  and  fecimdity  oc- 
curs. This  is  quite  similar  to  the  result  observed  in  the  case 
of  Drosophila,  and  is  quite  in  harmony  with  the  results  ob- 
tained by  ShuU  in  maize;  it  indicates  that  by  careful  selection 
races  may  be  secured  which  are  vigorous  in  spite  of  inbreed- 
ing, but  that  nevertheless  an  added  stimulus  to  growth  and 
reproduction  may  be  secured  in  such  cases  by  crossbreeding. 

In  the  production  of  pure  breeds  of  sheep,  cattle,  hogs,  and 
horses  inbreeding  has  frequently  been  practiced  extensively, 
and  where  in  such  cases  selection  has  been  made  of  the  more 
vigorous  offspring  as  parents,  it  is  doubtful  whether  any 
diminution  in  size,  vigor,  or  fertility  has  resulted.  Never- 
theless it  very  frequently  happens  that  when  two  pure  l)reeds 
are  crossed,  the  offspring  surpass  either  pure  race  in  size  and 
vigor.    This  is  the   reason  for  much  crossbreeding   in  eeo- 


232  GENETICS  AND  EUGENICS 

nomic  practice,  the  object  of  which  is  not  the  production  of  a 
new  breed,  but  the  production  for  the  market  of  an  animal 
maturing  quickly  or  of  superior  size  and  vigor.  The  inbreed- 
ing practiced  in  forming  a  pure  breed  has  not  of  necessity 
diminished  vigor,  but  a  cross  does  temporarily  (that  is  in  the 
Fi  generation)  increase  vigor  above  the  normal.  Now  why 
should  inbreeding  unattended  by  selection  decrease  vigor, 
and  crossbreeding  increase  it.^  We  know  that  inbreeding 
tends  to  the  production  of  homozygous  conditions,  whereas 
crossbreeding  tends  to  produce  heterozygous  conditions. 
Under  self-pollination  for  one  generation  following  a  cross 
(involving  one  unit-character  only),  half  the  offspring  become 
homozygous;  after  two  generations,  three-quarters  of  the 
offspring  are  homozygous;  after  three  generations  seven- 
eighths  are  homozygous,  and  so  on.  So  if  the  closest  inbreed- 
ing is  practiced  there  is  a  speedy  return  to  homozygous,  pure 
racial  conditions.  We  know  further  that  in  some  cases  at 
least  heterozygotes  are  more  vigorous  than  homozygotes. 
The  heterozygous  yellow  mouse  is  a  vigorous  lively  animal; 
the  homozygous  yellow  mouse  is  so  feeble  that  it  perishes  as 
soon  as  produced,  never  attaining  maturity.  Crossbreeding 
has,  then,  the  same  advantage  over  close  breeding  that  fer- 
tilization has  over  parthenogenesis.  It  brings  together  differ- 
entiated gametes,  which,  reacting  on  each  other,  produce 
greater  metabolic  activity.  East  and  Jones  have  suggested 
that  the  superiority  in  vigor  of  crossbred  over  inbred  indi- 
viduals is  roughly  proportional  to  the  number  of  genetic  dif- 
ferences between  the  races  crossed.  This  idea  is  worthy  of 
an  experimental  test. 

Inbreeding,  also,  by  its  tendency  to  secure  homozygous 
combinations,  brings  to  the  surface  latent  or  hidden  reces- 
sive characters.  If  these  are  in  nature  defects  or  weak- 
nesses of  the  organism,  such  as  albinism  and  feeble-minded- 
ness  in  man,  then  inbreeding  is  distinctly  bad.  Existing 
legislation  against  the  marriage  of  near-of-kin  is,  therefore, 
on  the  whole,  biologically  justified.  On  the  other  hand, 
continual  crossing  only  tends  to  hide  inherent  defects,  not  to 


INBREEDING  AND  CROSSBREEDING  233 

exterminate  them;  and  inbreeding  only  tends  to  bring  them 
to  the  surface,  not  to  create  them.  We  may  not,  therefore, 
Hghtly  ascribe  to  inbreeding  or  intermarriage  the  creation  of 
bad  racial  traits,  but  only  their  manifestation.  Further,  any 
racial  stock  which  maintains  a  high  standard  of  excellence 
under  inbreeding  is  certainly  one  of  great  vigor,  and  free  from 
inherent  defects. 

The  animal  breeder  is  therefore  amply  justified  in  doing 
what  human  society  at  present  is  probably  not  warranted  in 
doing,  —  viz.,  in  practicing  close  inbreeding  in  building  np 
families  of  superior  excellence  and  then  keeping  these  pure, 
while  using  them  in  crosses  with  other  stocks.  For  an  animal 
of  such  a  superior  race  should  have  only  vigorous,  strong  off- 
spring if  mated  with  a  healthy  individual  of  any  family  what- 
ever, within  the  same  species.  For  this  reason  the  production 
of  *' thoroughbred"  animals  and  their  use  in  crosses  is  both 
scientifically  correct  and  commercially  remunerative. 

The  early  plant  hybridizers  found  that  frequently  (but  not 
always)  hybrids  produced  by  the  crossing  of  distinct  species 
or  genera  are  characterized  by  remarkably  vigorous  growth 
and  large  size,  superior  to  that  of  either  parent.  But  these 
same  large  vigorous  hybrids  produced  little  or  no  seed. 
Vegetative  and  reproductive  activity  are  to  some  extent 
complementary  and  opposed  activities  of  the  plant.  A  vig- 
orously growing  young  fruit  tree  may  be  brought  into 
bearing  early  if  it  is  cut  partly  in  two,  or  a  ring  of  bark  is 
removed  from  it  in  the  growing  season,  thus  checking  its 
growth.  Under  such  circumstances  fruit  buds  are  formed. 
In  many  hybrid  plants,  in  which  the  vegetative  vigor  is  great, 
partial  or  complete  sterility  exists.  This,  however,  is  not 
invariably  the  case.  The  offspring  of  a  cross  between  geo- 
graphic varieties  of  the  same  species  are  usually  both  vigor- 
ous and  fertile,  but  the  offspring  of  widely  separated  species 
or  genera  may  be  lacking  in  vigor  as  well  as  fertility.  A\  ith 
increasing  diversity  of  the  parents  the  following  series  of 
conditions  obtains: 

1.  The  mating  of  parents  belonging  to  the  same  pure  race 


234  GENETICS  AND  EUGENICS 

and  closely  related  to  each  other  has  on  the  whole  the  same 
effect  as  seK-fertilization.  It  brings  together  gametes  which 
transmit  the  same  characters,  which  are  doubtless  chemi- 
cally alike,  and  no  particular  increase  of  vigor  results  when 
they  unite.  It  is  on  a  par  with  asexual  reproduction  by  par- 
thenogenesis, fission,  budding,  or  vegetative  multiplication. 
There  is  in  consequence  no  change  in  the  germinal  constitu- 
tion, or  relatively  little.  There  is  neither  increase  of  vigor 
nor  loss  of  vigor. 

2.  The  mating  of  closely  related  individuals  within  a  nor- 
mally intercrossing  population  such  as  a  breed  of  domesti- 
cated animals,  or  a  human  population,  is  apt  to  cause  some 
loss  of  vigor.  So  much  of  the  vigor  of  the  population  as  is 
due  to  its  crossed  (or  heterozygous)  character,  will  tend 
gradually  to  disappear,  as  homozygous  conditions  are  ob- 
tained in  consequence  of  inbreeding.  The  greater  the  num- 
ber of  characters  in  which  a  population  varies,  the  slower  will 
be  the  attainment  of  a  fully  homozygous  state  in  consequence 
of  inbreeding.  If  suflScient  vigor  is  retained  after  a  fully 
homozygous  state  has  been  reached,  then  the  closest  inbreed- 
ing (or  even  self-fertilization,  when  this  is  possible)  should 
cause  no  further  loss  of  vigor.  There  is  no  reason  to  think 
that  monstrosities  are  produced  by  inbreeding  (as  for  ex- 
ample deformities,  feeble-mindedness,  insanity)  except  in 
so  far  as  such  maladies  may  be  due  (1)  to  the  lack  of  suffi- 
cient vigor  on  the  part  of  the  organism  to  complete  its  normal 
development,  or  (2)  to  the  appearance  in  a  homozygous  state 
of  a  recessive  condition  unseen  in  the  heterozygous  parents. 

3.  The  mating  of  individuals  belonging  to  distinct  geo- 
graphical races  of  the  same  species  of  animal  or  plant  usually 
produces  offspring  larger  or  more  vigorous  than  either  parent 
and  fully  fertile.  The  same  result  follows  when  distinct  breeds 
of  domesticated  animals  or  distinct  varieties  of  cultivated 
plants  are  crossed.  The  offspring  are  equal  to  or  superior  to 
the  parents  in  vigor  and  not  less  uniform  in  character. 
But  the  F2  generation  from  such  a  cross  does  not  retain  the 
superiority  of  the  Fi  generation,  for  it  shows  great  variabil- 


INBREEDING  AND  CROSSBREEDING  235 

ity  in  all  respects,  which  in  economic  animals  or  plants  is 
very  undesirable.  For  the  characters  in  which  the  two  pure 
breeds  differed  undergo  recombination  in  all  possible  ways 
in  the  F2  offspring.  Even  a  back-cross  of  an  Fi  iiullvidujil 
with  one  of  the  pure  races  would  produce  offspring  (juite 
variable  and  including  undesirable  combinations,  since  each 
Fi  individual  would  form  the  maximum  number  of  different 
kinds  of  gametes.  Hence  crossing  of  pure  breeds  of  domes- 
ticated animals  may  in  special  cases  be  advantageous  but 
should  never  be  carried  beyond  the  Fi  generation  unless  the 
breeder  is  setting  out  on  the  slow  and  tedious  process  of  pro- 
ducing and  fixing  a  wholly  new  breed.  In  that  case  he  must 
be  prepared  to  produce  and  sacrifice  many  worthless  aninuils 
for  the  sake  of  obtaining  in  the  end  a  few  of  possibly  superior 
value.  For  such  an  undertaking  the  imagination  and  the 
patience  of  an  inventor  are  required. 

4.  When  animals  or  plants  of  widely  separated  species  or 
genera  are  crossed,  one  of  two  results  follows:  Either  the 
offspring  are  of  remarkable  vigor  but  of  impaired  fertility, 
or  the  offspring  lack  both  vigor  and  reproductive  capacity. 
In  the  former  category  comes  one  very  unportant  economic 
cross,  that  of  the  horse  with  the  ass,  producing  a  very  valu- 
able animal,  the  mule.  The  economic  importance  of  mules 
is  indicated  by  the  large  numbers  produced  in  the  United 
States,  South  America,  Europe  and  Africa,  and  by  the  fact 
that  the  market  price  of  a  mule  averages  higher  than  the 
price  of  either  a  horse  or  an  ass.  Nevertheless  a  mule  is 
absolutely  incapable  of  reproduction.  It  has  well  developed 
sexual  glands  and  sexual  instincts,  but  the  sexual  cells  de- 
generate before  reaching  full  maturity.  If  mules  were  capable 
of  reproduction,  they  would  probably  be  less  valued  than 
they  now  are,  for  F2  and  F3  individuals  would  doubtless  then 
be  produced,  and  these  would  lack  the  uniformity  and  vigor 
of  the  Fi  individuals  which  alone  exist  at  present. 

Crosses  of  cattle  with  the  American  bison  produce  hybrids 
which  are  sterile  in  the  male  sex  only,  the  femal(\s  being  fertile 
with  either  parent  species.    By  use  of  these  fertile  female 


236  GENETICS  AND  EUGENICS 

hybrids,  three-fourths  bloods  may  be  produced  which  are 
almost  as  variable  as  a  true  F2  generation.  If  the  products 
of  this  cross  are  shown  to  possess  economic  advantages  over 
domestic  cattle  (which  seems  very  doubtful)  a  fertile  hybrid 
race  will  doubtless  be  established  in  the  near  future.  How 
this  can  be  done  is  shown  in  experiments  made  by  Dr.  Det- 
lefsen  and  myself  in  crossing  the  guinea-pig  with  a  wild 
Brazilian  species  of  cavy,  Cavia  rufescens.  The  Fi  individuals 
surpass  either  parent  species  in  size  and  vigor,  but  the  males 
are  fully  sterile,  the  females,  however,  being  fertile.  After 
two  back-crosses  of  female  hybrids  with  the  guinea-pig  a 
few  fertile  males  were  obtained,  whose  descendants  were  also 
fertile.  But  they  possess  certain  Mendelizing  characters  de- 
rived from  the  wild  parent,  Cavia  rufescens.  The  skeletal 
characters  of  the  hybrids  are  a  blend.  The  great  vigor  of  the 
Fi  hybrids  is  not  shown  in  the  fertile  hybrids  obtained  by 
back-crossing.  As  regards  size  and  vigor  they  are  not  su- 
perior to  guinea-pigs.  If  the  Mendelizing  color  characters 
possessed  economic  value,  the  hybrid  race  could  now  be 
easily  continued.  As  in  the  case  of  the  cattle-bison  cross,  the 
economic  value  of  the  Fi  generation  is  not  sufficient  to  war- 
rant the  expense  of  its  continued  production. 

Hybrids  which  are  feeble  as  well  as  sterile  have,  of  course, 
no  economic  value.  They  are  scientifically  interesting  as 
showing  how,  when  the  difference  between  gametes  becomes 
too  great,  they  can  no  longer  form  a  vigorous  zygote.  Few, 
if  any,  animal  hybrids  of  this  sort  are  known,  but  many 
plant  hybrids  of  this  sort  have  been  produced,  among  them 
being  some  of  the  first  produced  hybrids  obtained  by  cross- 
ing different  species  of  Nicotiana  (tobacco).  See  Fig.  26a, 
East's  repetition  of  Kolreuter's  pioneer  experiment. 

5.  WTien  organisms  are  crossed  which  differ  more  widely 
than  do  ordinary  species,  so  that  they  are  referable  to  differ- 
ent genera  or  families,  the  production  of  a  hybrid  organism 
does  not  follow,  apparently  because  the  uniting  gametes  are 
too  unlike  to  be  capable  of  continued  existence  together  in  the 
same  cell.  Nevertheless  a  parthenogenetic  development  of  the 


INBREEDING  AND  CROSSBREEDING  237 

egg-cell  may  result  from  its  fertilization  by  the  foreign  sp<Tin. 
Thus  when  the  egg  of  a  sea  urchin  is  fertilized  with  the  sperm 
of  a  sea  lily,  an  anhnal  of  a  wholly  different  class  of  echino- 
derms,  the  egg  begins  development  following  a  fusion  of  the 
sperm  and  egg  nuclei,  but  the  nuclear  substance  introduced 
by  the  sperm  soon  degenerates  and  disappears.  The  eggy 
however,  having  once  started  to  develop,  continues  to  do  so, 
producing  an  organism  showing  only  characters  of  the  ma- 
ternal species.  Its  development  is  as  truly  parthenogenetic 
as  when  Induced  by  chemical  or  osmotic  means,  as  Is  now 
known  to  be  possible  in  the  case  of  the  eggs  of  many  marine 
and  of  some  fresh-water  animals.  Thus  the  unfertilized  egg 
of  a  frog  may  be  made  to  develop  by  chemical  means  (or 
even  by  puncturing  the  superficial  layer  of  the  egg  with  a 
needle),  a  process  we  may  call  artificial  or  induced  partheno- 
genesis. Now  in  crosses  of  species  too  widely  separated  to 
produce  a  hybrid  Individual,  the  sperm  may  merely  induce 
parthenogenesis.  This  method  of  inducing  parthenogenesis 
is  being  used  by  plant  breeders  of  the  United  States  Depart- 
ment of  Agriculture  to  obtain  orange  seedlings  which  It  Is 
hoped  may  be  superior  to  the  mother  plant  In  certain  re- 
spects, though  the  progeny  will  inherit  none  of  the  qualities 
of  the  pollen  plant.  It  Is  hoped  merely  that  there  may  occur 
in  the  parthenogenetic  offspring  some  segregations  or  vari- 
ations of  the  characters  found  in  the  mother  plant. 

What  might  be  called  male  parthenogenesis  has  been  re- 
ported In  crosses  of  strawberries  made  many  years  ago  by 
Millardet  and  also  In  a  cross  between  Mexican  teoslnte,  a 
plant  related  to  maize,  and  a  coarse  grass  of  the  southern 
United  States.  (Collins.)  In  such  cases  a  cross-fertilized  seeii 
produces  a  plant  which  shows  only  characters  of  the  pollen 
parent.  It  is  supposed  that  the  egg  nucleus  has  taken  no 
part  in  the  production  of  an  embryo,  but  that  this  has  arisen 
wholly  from  nuclear  material  of  the  pollen  tube. 

Considering  all  the  facts,  changes  in  heterozygosity  alone 
seem  an  insufficient  explanation  of  the  effects  of  crossing  and 
inbreeding  respectively.    It  is  necessary  to  suppose  further 


238  GENETICS  AND  EUGENICS 

that  gametes  as  well  as  zygotes  vary  in  vigor.  Some  can 
exist  as  gametes  alone,  so  great  is  their  natural  vigor.  Here 
there  can  be  no  heterozygosity.  Examples  are  found  both 
in  animals  and  in  plants  (honeybee  drone,  fern  gameto- 
phyte).  Others  can  exist  only  as  zygotes,  so  feeble  are  they 
(the  majority  of  the  higher  animals  and  plants).  Still  others 
cannot  exist  as  homozygotes,  but  only  as  heterozygotes,  be- 
cause they  are  still  feebler  (the  yellow  mouse,  the  aurea  snap- 
dragon). 

The  experience  of  Miss  King  in  inbreeding  rats  brother 
with  sister  for  twenty-five  generations,  shows  that  heterozy- 
gosity is  not  indispensable  to  vigor  even  in  bisexual  repro- 
duction, for  she  did  not  observe  any  evidences  of  decline  in 
vigor,  size  or  fecundity,  yet  in  all  probability  great  increase 
in  homozygosity  took  place,  since  variability  decreased. 

Pearl  (1915)  has  attempted  to  devise  a  precise  measure 
of  inbreeding  based  on  the  number  of  times  that  the  same 
individual  or  individuals  appear  in  the  pedigree  of  a  particu- 
lar animal.  Thus,  in  bi-parental  reproduction  each  individual 
has  two  parents,  each  of  these  also  had  two  parents,  which 
may  or  may  not  be  the  same  pairs.  If  the  parents  were 
brother  and  sister,  then  their  parents  were  one  pair,  not  two. 
Thus  the  maximum  number  of  different  ancestors  would  be 
two  parents,  four  grandparents,  eight  great-grandparents, 
etc.  Such  would  be  the  condition  when  no  inbreeding  had 
occurred.  But  occurrence  of  the  same  individual  more  than 
once  in  a  pedigree  would  show  a  certain  amount  of  inbreed- 
ing, and  the  extent  of  the  inbreeding  would  increase  with 
every  repetition  of  an  individual  in  the  pedigree.  Pearl 
makes  this  the  basis  of  his  "coefficient  of  inbreeding,"  which 
is  intended  to  express  the  relation  between  the  possible  (max- 
imum) number  of  different  ancestors  and  the  actual  number 
of  different  ancestors,  each  individual  being  counted  only 
once,  no  matter  how  many  times  it  is  mentioned  in  the 
pedigree. 

The  chief  utility  of  such  a  coefficient  is  to  show  what  ap- 
proach to  homoz^^gosity  of  genetic  factors  has  probably  been 


INBREEDING  AND  CROSSBREEDING  239 

made  in  the  production  of  a  particular  individual,  as  a  con- 
sequence of  mating  together  related  individuals  among  his 
direct  ancestors,  but  this  the  coefficient  of  inbreeding  can 
not  do  with  great  exactness  because  even  with  the  closest 
possible  inbreeding  (self-fertilization)  the  approach  to  honuj- 
zygosity  in  individual  cases  is  quite  a  matter  of  chance. 
Thus,  East  and  Jones  say,  forcefully  and  quite  correctly, 
*'The  rate  at  which  complete  homozygosity  is  approached 
depends  on  the  constitution  of  the  individuals  chosen. 
Theoretically  in  any  inbred  generation  the  progenitors  of 
the  next  generation  may  be  either  completely  heterozygous 
or  completely  homozygous  or  any  degree  in  between,  de- 
pending upon  chance.  The  only  condition  which  must  fol- 
low in  self-fertilization  is  that  no  individual  can  ever  be  more 
heterozygous  than  its  parent,  but  may  be  the  same  or  less. 
Thus  it  is  seen  that  artificial  inbreeding,  as  it  is  practiced, 
may  theoretically  never  cause  any  reduction  in  heterozygos- 
ity, or  it  may  bring  about  complete  homozygosity  in  the  first 
inbred  generation.  In  other  words  the  rate  at  which  homo- 
zygosity is  approached  may  vary  greatly  in  different  lines." 
.  .  .  ** Although  nearly  complete  homozygosis  is  theoreti- 
cally brought  about  by  seven  generations  of  self-fertilization, 
the  attainment  of  absolute  homozygosity  is  a  difficult  matter 
and  in  practice  it  may  never  be  reached."  .  .  .  "Con- 
tinued selective  mating  is  necessary  to  bring  about  homo- 
zygosity. Intermittent  inbreeding  alternating  with  periods 
of  outcrossing,  which  is  the  prevailing  state  of  affairs  ^^•ith 
many  organisms,  cannot  maintain  any  high  degree  of  homo- 
zygosity." These  statements  show  that  the  coefficient  of  in- 
breeding, though  it  appears  to  be  very  precise,  like  Galton's 
law  of  ancestral  heredity,  is  subject  to  similar  limitations. 
It  indicates  what  is  true  of  populations  in  the  mass,  but  has 
small  utility  as  an  indicator  of  what  happens  in  individual 
cases.  \Mien  it  is  applied  to  the  case  of  a  particular  Jersey 
bull  it  may  be  very  much  less  reliable  as  an  index  of  prob- 
able performance  than  the  judgment  of  an  experienced  cattle 
breeder. 


240 


GENETICS  AND  EUGENICS 


It  is  nevertheless  of  value  to  know  what  the  tendency  of  a 
particular  system  of  breeding  is,  if  persistently  followed,  as 
regards  homozygosity,  for  homozygosity  implies  fidelity  to 
type  in  transmission  and  is  probably  what  the  animal  breeder 
means  by  "prepotency,"  so  far  as  he  has  any  clearly  defined 


TABLE  32a 

Probable  Percentage  of  Homozygosity,  under  Different 

Systems  of  Inbreeding,  in  Populations  at  the  Outset  Entirely 

Heterozygous.     (From  Data  of  Jennings  and  Fish.) 


Generations  of 
Inbreeding 

Self-fertilization,  or 

Back -cross  to  the 

same  homozygous 

Parent 

Brother-sister 
Matings 

Generations  of 
Inbreeding 

Brother-sister 
Matings 

1 

50.00 

50.00 

12 

94.31 

2 

75.00 

50.00 

13 

95.40 

3 

87.50 

62.50 

14 

96.28 

4 

93.75 

68.75 

15 

96.99 

5 

96.87 

75.00 

16 

97.56 

6 

98.43 

79.69 

17 

98.03 

7 

99.22 

83.59 

18 

98.40 

8 

99.61 

86.72 

19 

98.71 

9 

99.80 

89.26 

20 

98.96 

10 

99.90 

91.31 

21 

99.15 

11 

.... 

92.97 

25 

99.64 

idea  in  mind  when  he  uses  the  term.  Inbreeding  tends  auto- 
matically to  replace  heterozygous  germinal  conditions  by 
homozygous  conditions  in  the  inbred  population  and  the 
*' closer"  the  degree  of  inbreeding  the  stronger  is  this  tend- 
ency. Jennings  has  worked  out  formulae  for  calculating  the 
probable  percentage  of  homozygosity  in  populations  inbred 
after  a  particular  system  of  matings  for  any  number  of  gen- 
erations. The  results  in  three  systems  of  matings  for  a 
series  of  from  10  to  25  inbred  generations  are  sho^n  in 
Table  32a.  The  progress  toward  homozygosis,  it  will  be  ob- 
served is  rapid  in  self-fertilization,  heterozygotes  being  only 
one-tenth  of  one  per  cent  after  10  generations  of  inbreed- 
ing.  The  elimination  of  heterozygotes  is  equally  rapid  when 


INBREEDING  AND  CROSSBREEDING  241 

back-crosses  are  made  in  every  generation  with  the  same 
homozygous  parent  race.  In  brother-sister  matings,  the  next 
nearest  degree  of  inbreeding,  progress  toward  homozygosity 
is  much  slower,  twenty-five  generations  of  sucli  matings 
accomplishing  no  more  than  eight  generations  of  self- 
fertilization. 


CHAPTER  XXVII 


HYBRID  VIGOR  OR  HETEROSIS 

Plants  or  animals  which  maintain  normal  size  and  vigor 
mider  self-fertilization  or  close  inbreeding  may  nevertheless 
show  an  added  vigor  when  outcrossed,  that  is  when  mated 
with  individuals  of  races  genetically  different  from  their  own. 
This  is  called  heterosis,  because  it  is  supposedly  due  to 
heterozygosis i  the  cross-bred  state  of  genetic  factors.  The 
mule  has  already  been  mentioned  as  a  familiar  example 
among  animals,  in  which  hybrid  vigor  is  shown.  Many 
similar  examples  are  on  record  for  hybrid  plants.  For  ex- 
ample East  and  Hayes  describe  a  cross  between  two  dif- 
ferent wild  varieties  of  tobacco  (Nicotiana  rustica  hrazilia 
and  N,  rustica  scahra)  showing  that  reciprocal  crosses  pro- 
duce Fi  plants  taller  than  either  parent  variety.    See  Table 

TABLE  326 

Variation  in  Height  of  Plants  of  Nicotiana  rustica  brazilia  (349),  of  N.  rustica 

scahra  (352),  and  of  their  Reciprocal  Fi  Hybrids 

{After  East  and  Hayes) 


Variety  or  cross 

Class  centers  in  inches 

24 

27 

30 

33 

36 

39 

42 

45 

48 

51 

54 

67 

60 

63 

66 

69 

72 

75 

78 

349 

4 

10 

22 

14 

7 

•  • 

2 

1 

5 

11 
1 

16 
3 

17 
0 
3 

6 
5 
5 

5 

2 

•    • 
•     • 

5 
4 

•     • 

6 
6 

•     • 

1 

5 

1 
1 

352 

352X349,  Fi 
349X352,  Fi 

•  • 

•  • 

•  • 

•  • 

•  • 

•  • 

2 

In  maize,  which  is  normally  cross-fertilized  and  so  main- 
tained as  a  field  crop  in  a  state  normally  heterozygous,  self- 
fertilization  for  a  number  of  generations  serves  automati- 
cally to  eliminate  most  of  the  heterozygosity  (see  Fig.  143) 
and  consequently  produces  races  of  size  and  vigor  less  than 


242 


Fig.  144.   Four  characteristically  different  inbred  strains  of  maize  after  eleven  generations  of  seiN 
fertilization.    Note  the  remarkable  uniformity  of  each  strain,    (\fter  East  and  Jones.) 


^^^PMj 

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Fig.  145,  Two  ears  of  maize  self -fertilized  for 
six  generations  and  between  them  an  ear  of 
their  Fi  hybrid,     (After  East  and  Jones.) 


Fig.  146.  Two  plants  of  maize  self-ferti- 
lized for  eleven  generations,  and  be- 
tween them,  plants  of  their  Fi  hybrid 
showing  greatly  increased  size  and  pro" 
ductiveness. 


Fig.  147.  A  field  of  maize  showing  remarkably  uniform  and  vigorou^  plants  representing  a  lirst  gener- 
ation cross  between  two  inbred  strains.     (After  Last  and  Jones.) 


100 


Gronth  Curves  of 
Two  Inbred  Strains 
of  Maize  and  Their 
P]^  and  Fg  Hybrids. 


to 

4) 

o 

c 


i 

A3 
00 


"T 

60 


90^ 


50  60  70  80 

Number  of  Days   from  Planting 

Fig.  148.  Graphs  showing  growth  curves  of  two  inbred  strains  of  maize,  P  (1-7)  and  P  (1-9),  ami  of 
their  Fi  and  F2  hybrids.  Note  that  both  Fi  and  F2  are  at  all  ages  much  taller  than  either  inbred  fwirent 
race,  but  that  Fi  is  considerably  taller  than  F2,  as  the  plants  approach  maturity.  (After  East  and 
Jones.) 


Pig.  152.  Diagram  showing  a  method  of  double  crossing  maize  to  secure  maximum  yield  from  »^i\ 
plot  and  general  crop.  Four  different  inbred  strains  (shown  in  the  top  row)  are  cros.s«'d  in  pairs,  pro- 
ducing the  two  vigorous  but  unrelated  Fi  hybrids  shown  in  the  middle  rt)w.  By  rros.sinK  Ihcso  with 
each  other,  an  entire  crop  of  Fi  seed  of  high  productiveness  is  secured.    (After  East  and  Jones.) 


fl 


HYBRID  VIGOR  OR  HETEROSIS  243 

normal  but  very  uniform  in  character  (Fig.  144).  But  if  two 
of  these  inbred  strains  are  crossed  with  each  other  a  great 
increase  ui  size  results  in  Fi,  which  as  it  accompanies  restora- 
tion of  the  origmal  heterozygosis  may  reasonably  be  ascribed 
to  its  agency  (see  Figs.  145-147).  If  a  second  generation 
of  the  crossed  corn  is  raised  by  planting  seeds  taken  from  Fi 
plants,  there  is  found  to  be  a  falling  off  in  vigor  fsee  Fig.  148). 
The  F2  plants  start  out  well  owing  to  the  large  amount  of 
food  materials  stored  in  the  plump  Fi  seeds,  but  ultimately 
they  fall  behind  Fi  plants  in  vigor  of  growth  so  that  they 
attain  a  height  considerably  less,  though  still  much  in  excess 

WeigM 

in 
Grams 


800 

KfRaeeB. 

—  ^ 

-^ 

==^ 

=_^_ 

fiOO 

?  h-nre  B 

dOO 

^ 

^ 

^'  ^^^^ 

6  CutUri. 

200 

^ 

/^ 

=rn 

- 

?  CutUri 

^ 

■ 

AgeinJDaya    40  80  120  160  200  240  280  320  360 

Fig.  149.    Growth  curves  of  race  B  guinea-pigs  and  of  Cavia  cutleri. 


-too 


of  the  inbred  parent  races.  This  is  in  harmony  wiLli  the 
view  that  heterosis  is  the  cause  of  hybrid  vigor,  for  heterosis 
should  be  at  a  maximum  in  Fi  and  should  decline  in  F2 
exactly  as  the  height  of  the  maize  plants  is  seen  to  do  in  this 
cross.  A  case  in  which  ordinary  (blending)  size  mheritimci' 
is  complicated  by  heterosis  is  seen  in  crosses  made  between 
Cavia  Cutleri  from  Peru  and  races  of  guinea-pigs  which  ^^  e 
will  call  B  and  C.  The  growth  curve  of  each  of  the  parent 
stocks  is  shown  in  Fig.  149.  In  each  case  males  are  heavier 
than  females  except  for  the  first  few  weeks  of  life  when  the 
females  are  heavier.  Races  B  and  C  are  nearly  twice  as  heavy 
in  adult  weight  as  Cavia  Cutleri. 

Growth  curves  of  the  Fi  and  Fo  hybrids  are  shown  in  Figs. 


244 


GENETICS  AND  EUGENICS 


150  and  151,  where  they  can  be  compared  with  the  growth 
curves  of  the  respective  parent  races.  In  each  case  Fi  sur- 
passes either  parent  race  in  size,  but  F2  is  intermediate  be- 
tween them.  So  far  as  heredity  is  concerned,  the  inheritance 
is  blending,  but  Fi  shows  an  increase  in  size  due  to  hybridi- 
zation. It  seems  to  be  due  not  to  heredity  at  all,  strictly 
speaking,  but  to  heterosis,  and  it  begins  to  disappear  as  the 
Fi  hybrids  are  bred  together  producing  an  F2  which  theo- 
retically is  only  half  as  heterozygous  as  Fi  (Table  32a).  It 
might  be  expected  to  decline  still  more  in  later  generations. 


Weight 

in\ 
GraMs 


A^e  iaDaya  40  80  120lfi0200240280320360«)a 

Fig.  150.   Growth  curves  of  race  B  and  Cavia  cutleri  males  and  of  their  Fi  and  F2  male  hybrids. 

Animal  breeders  have  long  utilized  the  principle  of  hetero- 
sis in  the  production  of  mules  and  in  the  "grading"  of  cattle, 
hogs,  and  sheep  for  meat  production.  Plant  breeders  are  like- 
wise seeking  to  take  advantage  of  this  same  principle  for  im- 
proving field  crops  in  quantity,  quality  and  uniformity  of 
yield.  In  particular  East  and  Jones  have  suggested  the  fol- 
lowing novel  methods  of  breeding  maize.  First,  a  standard 
variety  should  be  inbred  (self -pollinated)  for  several  gen- 
erations, in  the  course  of  which  it  will  automatically  resolve 
itself  into  a  number  of  genetically  different  pure  lines  (Fig. 
144).  Any  lines  inherently  weak  w^ill  become  extinct  or  may 
be  discarded.  Those  which  remain  will  contain  the  best  com- 
binations of  genetic  factors  originally  present  in  the  variety, 
but  will  lack  any  vigor  due  to  heterosis  and  so  will  be  less 


HYBRID  VIGOR  OR  HETEROSIS 


24 


o 


productive  than  the  original  variety  before  it  was  inbred. 
It  will  accordingly  not  be  profitable  to  propiigate  these  pure 
lines  as  field  crops,  and  further  the  amount  of  seed  w]iif]i 
they  will  yield  if  cross-pollinated  will  not  be  large.  Hence  to 
produce  a  large  quantity  of  cross-bred  seed  will  be  expensive. 
But  from  a  small  number  of  Fi  plants  a  very  large  yield  of  F. 
seed  might  be  obtained  at  small  expense,  since  Fi  plants  are 
extremely  productive.  The  aim  should  be  therefore  to  cross- 
breed Fi  plants.  This  can  be  done  by  securing  four  different 
inbred  lines  and  crossing  these  in  pairs,  A  with  B,  C  with  D. 
There  will  result  two  unrelated  and  vigorous  Fi  groups,  AB 
and  CD  which  may  now  be  planted  in  alternate  rows.    One 


Weight 

in  I 
Orams 


AgeinDaya  40  80  120  160  200  240  2S0  320  360  400 

Fig.  151.  Growth  curves  of  race  B  and  Cavia  cutleri  females  and  their  Fi  and  F2  female  hybrids. 

of  them,  if  detasseled,  will  be  naturally  pollinated  by  the 
other  and  consequently  all  the  seed  which  it  produces  will 
be  crossbred,  representing  combinations  of  factors  found  in 
AB  with  the  allelomorphic  factors  in  CD.  Such  seed,  if 
planted,  will  produce  a  field  crop  of  maximum  yield,  since 
all  plants  will  be  cross-bred  Fi  individuals,  though  produced 
by  Fi  plants.  This  last  fact  will  keep  down  the  cost  of  pro- 
ducing the  seed  because  the  yield  will  be  heav>%  half  the 
total  crop  from  the  area  planted  (see  Fig.  152). 


CHAPTER  XXVIII 

GALTON'S  LAW  OF  ANCESTRAL  HEREDITY  AND  HIS 
PRINCLPLE  OF  REGRESSION 

Galton  (1889)  was  the  first  to  recognize  the  distinction  be- 
tween alternative  and  blending  inheritance.  But  he  sought 
nevertheless  to  unify  the  two  categories  of  cases  and  finally 
formulated  in  1897  a  generalized  *'law  of  ancestral  heredity" 
which  he  believed  would  include  both.  In  seekfng  such  a 
general  law  of  heredity  he  had  studied  a  representative  case 
each  of  blending  and  of  alternative  inheritance.  The  former 
was  found  in  family  statistics  of  human  stature,  the  latter  in 
the  coat  color  of  Basset  hounds.  The  latter  we  should  now 
describe  as  a  case  of  Mendelian  inheritance  involving  simul- 
taneously white  spotting,  and  a  color  pattern  (bi-color). 
Stature  inheritance  is  well  described  by  Galton's  term, 
''blending,"  but  is  now  understood  to  involve  multiple 
Mendelian  factors  whose  action  is  cumulative. 

In  either  case,  Galton  would  have  admitted  that  the  entire 
inheritance  is  from  the  parents  through  the  two  gametes 
which  unite  to  form  the  zygote,  so  that  strictly  speaking 
there  is  no  inheritance  from  generations  more  remote  than 
the  parents.  But  he  w^ould  have  maintained  quite  correctly 
that  a  better  idea  can  be  had  of  what  the  gametes  on  the 
average  will  transmit,  if  one  knows  the  character  of  several 
generations  of  ancestors  than  if  one  knows  the  character  of 
the  parents  alone,  and  in  this  sense  we  may  be  said  to  inherit 
from  ancestors  more  remote  than  our  parents.  Galton  be- 
lieved that  the  apparent  influence  of  each  generation  of  an- 
cestors diminished  as  its  remoteness  increased,  each  more 
remote  generation  having  only  half  the  influence  of  the  next 
later  one.  In  his  own  words:  "The  two  parents  contribute 
between  them,  on  the  average,  one-half,  or  (0.5);  the  four 
grandparents,  one-quarter,  or  (0.5)  2;  the  eight  great-grand- 
parents, one-eighth,  or  (0.5) ^  and  so  on.    Thus  the  sum  of 

246 


GALTON'S  LAW  Q47 

the  ancestral  contributions  is  expressed  by  the  series  (0.5)  -f- 
(0.5)  2  -f  (0.5)  3,  etc.]  which  being  equal  to  1,  accounts  for 
the  whole  heritage." 

If  one  attempts  to  make  use  of  this  law  by  basing  upon  it 
predictions  as  to  the  character  of  the  offspring  in  particular 
kinds  of  matings,  it  works  fairly  well  when  blending  charac- 
ters are  tinder  consideration,  but  fails  completely  when  ordi- 
nary Mendehzing  characters  are  under  consideration.  See 
Castle  (1903).  As  a  useful  generalization  it  is  now  pretty 
generally  discredited. 

Regression  was  a  name  given  by  Galton  to  the  apparent 
going  back  of  offsprmg  from  the  condition  of  their  parents 
toward  that  of  more  remote  ancestors,  or  more  correctly 
toward  the  general  average  of  the  race.  Thus  he  observed  that 
very  tall  parents  have  children  less  tall  than  themselves, 
while  very  short  parents  have  children  taller  than  themselves. 
In  either  case  the  children  regress  toward  the  general  average 
of  the  race,  and  the  regression  is  greater  the  more  pronounced 
the  deviation  of  the  parents  from  the  general  average  of  the 
race.  Also  in  sweet  peas,  Galton  observed  that  when  very 
large  seeds  are  planted,  the  crop  harvested  averages  smaller 
in  size  than  the  seeds  planted;  and  that  when  small  seeds 
are  planted,  the  crop  averages  larger  in  size.  Regression 
occurs  in  both  cases  toward  the  mean  of  the  race.  Galton 
regarded  regression  as  a  feature  of  ancestral  heredity;  but 
Johannsen  has  shown,  as  regards  size  of  beans,  that  regres- 
sion is  due  to  a  lack  of  agreement  between  somatic  and 
genetic  variations,  the  latter  being  more  conservative,  ami 
that  when  selection  is  made  within  a  line  pure  genetically,  no 
regression  occurs.  Davenport  confirms  this  view  in  the  case 
of  human  stature,  showing  that  the  children  of  parents 
genetically  pure  for  tall  stature  do  not  regress  toward  me- 
diocrity, as  Galton  supposed  all  classes  of  a  population  to  do. 
Galton's  law  of  ancestral  heredity  and  his  principle  of  regres- 
sion are  now  chiefly  of  historical  interest,  but  it  is  well  to 
keep  them  in  mind  when  generalizations  based  on  similar 
reasoning  are  brought  forward.     (See  Chapter  XX^  I.) 


CHAPTER  XXIX 


SEX  DETERMINATION 


Certain  facts  presented  in  an  earlier  chapter  show  that  there 
is  a  close  connection  between  sex-linked  inheritance  and  sex 
determination,  since  only  male-determining  gametes  or  only 
female-determining  gametes  are  able  to  transmit  sex-linked 
characters  in  particular  crosses.  We  must  now  consider  more 
fully  the  facts  and  theories  of  sex  determination.  In  all  the 
higher  animals  and  plants  a  discontinuous  variation  occurs 
as  regards  sex,  every  individual  being  either  male  or  female. 
The  distribution  of  males  and  females  in  successive  genera- 
tions presents  many  analogies  with  Mendelian  inheritance. 
This  idea  occurred  to  Mendel  himself,  as  is  shown  in  his  post- 
humously published  letters.  Bateson  suggested  it  independ- 
ently in  1902,  and  this  idea  was  more  fully  elaborated  by 
Castle  (1903).  The  view  is  now  generally  accepted  that  a 
factor  concerned  in  sex  determination  is  in  all  the  higher 
animals  and  plants  inherited  in  accordance  with  Mendel's 
law.  What  in  such  cases  is  the  distinction  between  male  and 
female  individuals? 

The  essential  difference  between  a  female  and  a  male  indi- 
vidual is  that  one  produces  eggs,  the  other  sperm.  All  other 
differences  are  secondary  and  dependent  largely  upon  the 
differences  mentioned.  If  in  the  higher  animals  (birds  and 
mammals)  the  sex  glands  (i.  e.,  the  egg-producing  and  sperm- 
producing  tissues)  are  removed  from  the  body,  the  superficial 
differences  between  the  sexes  largely  disappear.  In  insects, 
however,  the  secondary  sex  characters  seem  to  be  for  the 
most  part  uninfluenced  by  presence  or  absence  of  the  sex 
glands.  Their  differentiation  occurs  independently,  though 
simultaneously,  with  that  of  the  sex  glands,  evidently  de- 
pending on  the  genetic  (chromosome)  constitution  of  the 
cells  in  each  part  of  the  body.  When  the  constitution  of  cells 

248 


Fig.  155.  Effects  of  removal  or  transplantation  of  sex  glands  in  Brown  Leghorn  fowls.  1  and  •?.  Normal 
male  and  female  respectively.  3.  Feminized  male.  At  an  early  age  the  testes  were  remove*!  anil  re- 
placed by  ovaries.  4.  Castrated  male,  three  years  old.  Notice  undevelop<Mi  comb  and  wattles,  but 
characteristic  male  hackle  feathers,  tail  feathers  and  spurs.  5.  Castrated  female.  Notice  well-developed 
comb  and  wattles  but  characteristic  female  plumage.    (After  Dr.  H.  D.  Goodale.) 


SEX  DETERMINATION 


249 


in  different  parts  of  the  body  differs  in  respect  to  sex-linked 
characters,  a  sex-mosaic  results  known  as  a  gynandroniorph. 
Morgan  and  Bridges  have  made  an  exhaustive  study  of  such 
mosaic  individuals  found  in  their  cultures  of  Drosophila. 
One  of  the  simplest  types,  a  bilateral  sex-mosaic,  is  shown  in 
Fig.  153.  The  right  half  of  this  fly  shows  male  characters, 
viz.,  shorter  wing,  black-tipped  abdo- 
men, sex-comb  on  first  leg.  The  left 
side  of  the  fly  shows  the  contrasted 
female  characters.  The  right  eye  was 
also  white,  a  character  inherited  in  the 
single  X-chromosome  derived  from 
the  white-eyed  mother  of  the  fly.  The 
left  eye  was  red  resulting  from  the 
presence  (in  the  female  part  of  the 
body)  of  an  X-chromosome  bearing 
red-eye,  derived  from  the  red-eyed 
father,  which  is  dominant  over  the 
white-eye  borne  by  the  X-chromosome 
furnished  by  the  mother. 

Three  different  explanations  have 
been  offered  in  recent  years  for  the 
origin  of  sex-mosaic  insects.  These  are 
expressed  diagram  mat  ically  in  (Fig. 
154).  The  first.  A,  was  offered  by 
Boveri.  It  suggests  that  an  egg  which 
has  undergone  maturation,  and  which 
accordingly  retains  a  single  X-chro- 
mosome may,  on  account  of  delayed 
fertilization,  undergo  nuclear  division 
before  fertilization  is  complete,  so  that 
it  becomes  binucleate  before  fusion  of  egg  and  sperm  nuclei 
has  occurred.  The  sperm  now  fuses  with  one  of  the  egg's  two 
nuclei.  That  nucleus  and  its  descendants  will  be  ^2X  (female), 
but  the  unfertilized  nucleus,  if  it  develops  by  itself  will  be 
X  (male).  A  body  mosaic  as  to  sex  will  result,  part  male, 
part  female.   The  case  shown  in  (Fig.  153)  could  be  accounted 


Fig.  153.  A  ^ex-mosaic,  or  gynan- 
dromorphic,  Drosophila.  The  rikfht 
half  of  the  body  shows  male  char- 
acters, viz.  comb  on  first  h'^r.  shi)rt 
wing,  and  black-tipped  ab<lonien. 
The  left  half  of  the  bo«ly  shows 
female  characters,  viz.  long  wing 
and  light-tipped  ab<lomcn.  Note 
also  that  the  right  eye  waa  white, 
the  left  eye  red.  See  text,  (.\fter 
Morgan.) 


250 


GENETICS  AND  EUGENICS 


for  on  this  hypothesis,  but  many  other  cases  in  Drosophila 
cannot,  for  which  reason  Morgan  and  Bridges  favor  a  differ- 
ent explanation.  B  and  C  (Fig.  154)  are  explanations  of  sex- 
mosaics  offered  at  different  times  by  Morgan.  In  B  it  is 
supposed  that  two  sperms  have  entered  the  egg,  one  of 
which  united  with  the  egg-nucleus  and  produced  a  female 
(2X),  hybrid  as  to  sex-linked  characters,  the  other  developing 
by  itself  produced  male  parts  showing  only  characters  of  the 
father.  This  explanation  evidently  will  not  fit  the  case  of 
(Fig.  153)  because  the  male  side  of  the  fly  inherits  from  the 


Fig.  154.   Three  different  explanations  which  have  been  offered  to  account  for  the  production  of  gynan- 
dromorphs  (sex-mosaics  or  sex-intergrades)  in  Drosophila.    See  text.    (After  Morgan.) 

mother,  not  the  father.  An  alternative  explanation,  C,  is 
offered  by  Morgan  for  such  cases  as  this.  It  is  supposed  that 
the  egg  has  been  normally  fertilized  but  that  in  a  division 
of  the  fertilized  nucleus,  one  division  product  of  an  X- 
chromosome  gets  left  behind  at  the  middle  of  the  spindle. 
Thus  one  daughter  nucleus  gets  two  X-chromosomes  (fe- 
male) and  the  other  only  one  (male).  Whether  the  male 
part  shows  maternal  or  paternal  characters  will  depend  on 
which  X-chromosome  (maternal  or  paternal)  was  eliminated. 
Explanation  C  is  thus  an  alternative  to  A  for  cases  in  which 
the  male  part  of  the  mosaic  shows  maternal  characters,  and  it 
also  affords  an  explanation  (alternative  to  B)  of  cases  in  which 
the  male  part  of  the  mosaic  shows  paternal  characters. 
In  contrast  to  the  case  of  insects,  the  dependence  of  second- 


SEX  DETERMINATION  0.5 1 

ary  sex  differences  in  mammals  and  birds  upon  the  presence 
of  the  gonads  acting  through  secretions  (hormones)  is  clearly 
shown  by  the  experimental  work  of  Steinach  and  Goodale. 
The  former  castrated  immature  male  rats  and  guinea- i)igs 
and  then  introduced  into  the  bodies  of  the  castrated  males 
ovaries  of  the  female  of  the  same  species.  The  transplanted 
ovaries  became  established  and  caused  remarkable  changes 
in  the  castrated  animals.  Their  mammary  glands,  which  are 
rudimentary  in  the  male,  became  greatly  enlarged.  The 
body  remained  small  as  in  females  and  the  fur  soft.  Their 
behavior  too  was  more  like  that  of  females  than  of  males. 

Goodale  (1916)  performed  a  similar  experiment  on  male 
brown  Leghorn  chicks  with  like  results.  (See  Fig.  155.) 
Goodale  (1911%  1913)  found  also  that  mere  removal  of  the 
ovaries  from  female  birds  (hens  and  ducks)  causes  them  to 
assume,  to  a  considerable  extent,  the  quite  different  appear- 
ance of  males  and  that  castrated  males  fail  to  develop  many 
of  the  normal  male  characteristics.  It  is  accordingly  clear 
that  some  secretion  of  the  ovary  normally  acts  as  an  inhibitor 
against  the  development  of  male  plumage  in  birds,  and  that 
in  males  a  secretion  of  the  testis  is  necessary  for  full  develop- 
ment of  the  secondary  sex  characters. 

Morgan  has  shown  that  what  in  female  fowls  acts  as  an 
inhibitor  to  the  development  of  male  plumage  is  not  a  secre- 
tion of  the  egg-cells  proper  but  a  secretion  of  certain  "luteiil 
cells"  normally  present  in  the  ovary.  He  finds  that  in  Se- 
bright bantams,  which  breed  has  hen-feathered  males, 
"luteal  cells"  are  present  in  the  testis  of  males  as  well  as  in 
the  ovary  of  females.  Consequently  when  Sebright  bantam 
males  are  castrated  they  become  ** cock-feathered,"  that  is 
they  grow  the  long  tail-feathers  and  the  hackle  feathers 
characteristic  of  males  in  other  breeds  of  fowls.  In  crosses 
of  Sebright  bantams,  in  which  the  cocks  are  hen-feathered, 
with  black-breasted  game  bantams,  in  which  the  cocks  are 
normal,  Morgan  found  that  hen-featliering  behaved  iis  a 
non-sex-linked  dominant  character  probably  involving  two 
distinct  genetic  factors. 


252  GENETICS  AND  EUGENICS 

To  recapitulate,  we  have  in  fowls  this  relationship  of 
plumage  and  other  secondar^^  sex  characters  to  the  gonads 
or  their  secretions.  Fowls  of  both  sexes  will  develop  the 
same  plumage  characters,  viz.,  the  full  plumage  of  normal 
males,  if  no  secretions  interfere.  In  females  such  an  inhibit- 
ing secretion  is  normally  produced  by  luteal  cells  present  in 
the  ovary,  and  in  hen-feathered  males  luteal  cells  in  the 
testis  produce  a  similar  secretion.  If  luteal  cells  are  intro- 
duced into  castrated  males  (in  transplanted  ovaries)  the 
birds  become  hen-feathered.  Likewise  if  the  luteal  cells 
are  removed  (with  the  ovary)  from  a  female,  she  becomes 
"cock-feathered."  If  the  luteal  cells  are  removed  (with  the 
testes)  from  a  hen-feathered  cock,  he  becomes  cock-feathered. 
Hence  ** hen-feathering"  in  either  sex  is  due  to  the  secretion 
of  luteal  cells,  not  to  the  sex-cells  proper.  But  the  developed 
condition  of  comb  and  wattles  normally  seen  in  males  is  due 
to  a  different  secretion  formed  by  the  testis.  For  this  con- 
dition disappears  in  castrated  males  and  is  not  attained  in 
feminized  males  into  which  ovaries  have  been  introduced. 

In  male  sheep  a  secretion  of  the  testis  seems  to  act  as  a 
stimulant  to  horn  development,  for  male  sheep  regularly  have 
larger  horns  than  females  (Fig.  97)  and  in  some  breeds,  for 
example  the  merino,  males  only  have  horns.  (See  Figs.  101 
and  103.)  Early  castration  of  the  male  in  such  breeds  results 
in  hornlessness. 

Finally  Lillie  (1916)  has  shown  that  in  cattle  hormones 
in  the  blood  of  the  developing  male,  if  allowed  to  enter  the 
circulation  of  the  developing  female,  so  interfere  with  the 
growth  of  the  ovary  as  to  render  its  possessor  sterile.  This 
is  the  explanation  of  the  *'free  martin,"  a  sterile  female  calf 
born  as  a  twin  to  a  male  calf.  The  twins  in  this  case  begin 
their  development,  each  from  a  separate  fertilized  egg,  but 
become  later  so  closely  crowded  together  in  the  uterus  of  the 
mother  that  their  foetal  blood  vessels  unite,  allowing  the 
blood  from  one  embryo  to  pass  freely  over  into  the  other. 
A  sterilizing  influence  on  the  female  results,  the  ova  in  the 
body  of  the  female  embryo  failing  to  grow,  but  no  reciprocal 


SEX  DETERMINATIOX  253 

influence  on  the  male  has  been  noted,  nor  is  the  sex  of  the 
female  changed  but  merely  her  sexual  development  repressed. 

An  interesting  case  of  sex  control  through  secretions  litis 
recently  been  discovered  in  a  mollusk,  Crepiduhi.  The  in- 
dividuals of  this  species  normally  function  as  males  when 
they  are  small,  at  that  time  developing  sperm,  but  when 
grown  to  larger  size  they  develop  eggs  and  function  as  fe^ 
males.  Since  eggs  and  sperm  are  not  developed  simultane- 
ously m  the  same  mdividual,  the  eggs  are  regularly  cross 
fertilized.  Gould  has  shown  that  if  a  smaU  Crepiduhi  is  iso- 
lated from  other  individuals  it  remains  a  "neuter,"  but  that 
if  it  is  brought  withui  a  few  millimeters  of  a  large  (female) 
individual,  it  proceeds  to  develop  as  a  male  and  liberates 
sperm.  The  action  is  supposed  to  result  from  substances 
given  off  into  the  sea  water  from  the  body  of  the  nearby 
female.  As  the  individuals  of  Crepidula  remain  in  one  i)lace 
practically  throughout  their  adult  life,  this  curious  adapta- 
tion has  manifest  advantages  to  the  species. 

The  egg  or  larger  gamete  (the  so-called  macro-gamete)  in 
all  animals  is  non-motile  and  contains  a  relatively  large 
amount  of  reserve  food  material  for  the  maintenance  of  the 
developing  embryo.  This  reserve  food  material  it  is  the  func- 
tion of  the  mother  to  supply.  In  the  case  of  some  animals, 
for  example  flatworms  and  mollusks,  the  food  supply  of  the 
embryo  is  not  stored  in  the  egg-cell  itself,  but  in  other  cells 
associated  with  it,  which  break  dow'n  and  supply  nourish- 
ment to  the  developing  embryo  derived  from  the  fertilized 
egg.  Again,  as  in  the  mammals,  the  embryo  may  derive  its 
nourishment  largely  from  the  maternal  tissues,  the  embryo 
remaining  like  a  parasite  within  the  maternal  body  during 
its  growth,  feeding  by  osmosis.  But  in  all  cases  alike  the 
mother  supplies  the  larger  gamete  and  the  food  material 
necessary  to  carry  the  zygote  through  its  embryonic  stages. 
The  father,  on  the  other  hand,  furnishes  the  bare  hereditary 
equipment  of  a  gamete,  with  the  motor  ai)paratus  nect^ssary 
to  bring  it  into  contact  with  the  egg-cell  but  without  food  for 
the  developing  embryo  produced  by  fertilization.    The  ga- 


254  GENETICS  AND  EUGENICS 

mete  furnished  by  the  father  is  therefore  the  smaller  gamete, 
the  so-called  micro-gamete. 

From  the  standpoint  of  metabolism,  the  female  is  the  more 
advanced  condition:  the  female  performs  the  larger  function, 
doing  all  that  the  male  does  in  furnishing  the  material  basis 
of  heredity  (a  gamete),  and  in  addition  supplying  food  for  the 
embryo.  As  regards  the  reproductive  function,  the  female  is 
the  equivalent  of  the  male  organism,  plus  an  additional  func- 
tion, —  that  of  supplying  the  embryo  with  food.  When  we 
come  to  consider  the  structural  basis  of  sex,  we  find,  often  in 
differences  in  chromosome  number,  reasons  for  thinking  that 
here,  too,  the  female  individual  is  the  equivalent  of  the  male 
plus  an  additional  element.^  The  conclusion  has  very  natu- 
rally been  drawn  that  if  a  means  could  be  devised  for  increas- 
ing the  nourishment  of  the  egg  or  embryo,  its  development 
into  a  female  should  be  thereby  insured,  while  the  reverse 
treatment  should  lead  to  the  production  of  a  male. 

In  a  few  cases  it  has  been  found  possible  by  indirect  means 
to  control  the  state  of  nutrition  of  the  eggs  and  so  to  control 
the  sex  of  the  individual  which  develops  from  it.  Thus  in  the 
rotifer,  Hydatina  senta,  parthenogenetic  eggs  of  two  sorts 
are  produced,  which  are  either  male-producing  or  female- 
producing,  the  former  being  smaller.  Whitney  has  shown 
that  when  a  colony  of  Hydatina  is  fed  for  a  generation  exclu- 
sively on  the  green  flagellate,  Dunaliella,  practically  all  the 
mothers  lay  male-producing  eggs,  but  a  continuous  diet  of 
the  colorless  flagellate,  Polytoma,  leads  to  the  production  of 
female  eggs.  The  effect  in  each  case  is  seen  not  in  the  first 
generation,  but  in  the  second  generation  of  offspring.  The 
female  fed  on  Dunaliella  has  grandsons;  the  female  fed  on 
Polytoma  has  granddaughters.  The  diet  of  the  mother  is 
immaterial. 

In  pigeons,  eggs  are  produced  in  clutches  of  two  each,  and 
in  wild  species  these  commonly  develop,  one  into  a  male,  the 

^  But  in  the  poultry  type  qf  sex -linked  inheritance  it  is  evident  that  the  male  is 
more  liberally  equipped  with  certain  genes,  in  which  he  is  duplex  while  the  female 
is  simplex. 


SEX  DETERMINATIOX  255 

other  into  a  female.  Riddle  has  shown  tliat  the  female-pro- 
ducing egg  is  the  larger  of  the  two  and  contains  the  larger 
amount  of  potential  chemical  energy.  If  the  eggs  are  re- 
moved from  the  nest  as  fast  as  laid,  the  female  is  induced  to 
lay  a  larger  number  of  eggs  than  she  would  otherwise  have 
laid  and  the  majority  of  these  are  female-producing.  Toward 
the  end  of  the  season  nothing  but  females  may  come  from 
eggs  the  production  of  which  is  forced  in  this  way. 

In  such  cases  sex  is  subject  to  a  certain  amount  of  conlnjl 
through  the  state  of  nutrition  of  the  egg  itself.  But,  neither 
in  this  case  nor  in  that  of  most  other  animals  is  the  state 
of  nourishment  of  the  single  eggs  directly  affected  by  nourish- 
ment of  the  mother. 

In  certain  cases  (Daphnia)  poor  nutrition  of  the  mother 
may  diminish  the  number  of  eggs  which  she  liberates,  without 
increasing  the  proportion  of  males  among  the  offspring  pro- 
duced, since  nourishment  of  the  individual  egg  is  not  lessened, 
for  the  eggs  under  such  circumstances  resort  to  cannibalism, 
devouring  one  another,  and  those  which  survive  are  fully 
nourished. 

Attempts  to  influence  the  sex  of  an  embryo  or  larva  by 
altered  nutrition  of  the  embryo  or  larva  itself  have  proved 
equally  futile.  Practically  the  only  experimental  evidence  of 
value  in  favor  of  this  idea  has  been  derived  from  the  study  of 
insects,  and  this  is  capable  of  explanation  on  quite  different 
grounds  from  those  w^hich  first  suggest  themselves.  It  Inis 
sometimes  been  observed,  as  by  Mary  Treat  for  example,  that 
a  lot  of  insects  poorly  fed  produce  an  excess  of  males.  In 
such  lots,  however,  the  mortality  is  commonly  high,  and 
more  females  die  than  males,  because  the  female  is  usually 
larger  and  requires  more  food  to  complete  her  develoi)ment. 

A  delayed  fertilization  of  the  egg  has  in  certain  ciises, 
notably  frog's  eggs,  been  shown  to  increase  the  i)ercentage  of 
male  offspring.  This  is  not  due  to  any  change  in  the  si)er- 
matozoa,  as  experiment  clearly  shows,  but  merely  to  the  rela- 
tive staleness  of  the  egg.  If  the  fertilization  of  the  frog's  egg 
is  delayed  three  or  four  days  after  its  passage  into  the  uterus, 


256  GENETICS  AND  EUGENICS 

more  male  offspring  occur.  It  is  possible  that  the  chemical 
composition  of  the  egg  changes  when  fertilization  is  delayed, 
the  total  energy  content  decreasing  and  so  diminishing  the 
probability  that  the  egg  will  develop  into  a  female.  Riddle's 
work  with  pigeons  suggests  such  an  interpretation. 

Further  frog's  eggs  may  by  various  means  be  caused  to 
develop  parthenogenetically.  Loeb  (1918)  has  raised  twenty 
leopard  frogs  from  unfertilized  eggs  artificially  stimulated 
into  development  by  the  prick  of  a  needle.  Among  these 
frogs  both  sexes  were  represented  and  the  chromosome  num- 
ber was  found  to  be  diploid.  Accordingly  sex  differentiation 
in  this  case  would  seem  not  to  have  depended  upon  chro- 
mosome reduction  of  the  ordinary  sort.  Again  King  (1912) 
has  shown  that  keeping  toad's  eggs  out  of  water  for  several 
hours  after  fertilization  raises  the  percentage  of  female 
young  from  fifty  to  over  seventy.  Hence  it  may  be,  as 
Riddle  thinks,  that  the  natural  sex  tendencies  of  the  gametes 
may  under  certain  conditions  be  overbalanced  or  counter- 
acted by  other  agencies  influencing  metabolism,  the  eggs 
perhaps  developing  parthenogenetically  without  passing 
into  the  haploid  chromosome  state  of  ordinary  gametes. 

What  are  we  to  understand  by  the  expression,  '*  natural  sex 
tendencies  of  the  gametes  "  ?  Obviously  what  is  meant  is  the 
genetic  constitution  of  the  gametes,  that  is  their  content  of 
genes.  But  we  have  seen  that  strong  reasons  exist  for  believ- 
ing that  genes  are  found  exclusively  in  the  chromatin.  If  this 
is  so,  "natural  sex  tendencies  of  the  gametes,"  can  mean  only 
composition  of  the  gametes  as  regards  chromatin.  One  of  the 
most  important  generalizations  reached  in  recent  years  by 
cytologists  is  this,  that  the  chromatin  composition  of  the 
gamete  does  in  reality  determine  its  natural  sex  tendencies. 

In  a  great  many  animals,  possibly  in  all,  the  chromosome 
composition  of  the  individual's  cell-nuclei  bears  an  interest- 
ing relation  to  its  sex.  Thus  in  bees,  ants,  wasps,  and  related 
insects,  as  well  as  in  small  Crustacea  and  rotifers,  only  females 
develop  from  fertilized  eggs,  i.  e.,  from  zygotes,  whereas  males 
develop  from  unfertilized  eggs  which  have  the  nuclear  con- 


SEX  DETERMINATION  / 


52.57 


stitution  of  gametes,  and  which,  in  some  cases  at  least,  are 
capable  of  actually  functioning  as  gametes.  It  would  seem 
that  in  such  cases  the  female  must  have  a  duplex  chromosome 
composition,  since  two  gametes  have  united  to  pnxhice  it, 
whereas  the  male  can  be  only  simplex,  since  he  represents 
a  developed  gamete. 

The  case  of  the  honeybee  affords  a  familiar  example,  l^ie 
mother  bee,  or  "queen  "of  the  hive,  lays  eggs  which  are 
capable  of  development  either  vnth  or  without  fertiHzatioii. 
The  mother  is  able  to  produce  or  to  withhold  fertlHzalion 
according  to  circumstances,  for  she  has  in  a  sac  connected 
with  the  oviduct  a  supply  of  sperm  received  at  mating.  The 
eggs  pass  the  outlet  of  this  sac  as  they  are  laid.  The  outlet 
of  the  sac  is  controlled  by  muscles  which  relax  when  an  egg 
is  to  be  fertilized,  permitting  sperm  to  come  in  contact  with 
the  egg,  but  closing  the  outlet  tightly  when  the  egg  is  not  to 
be  fertilized.  Fertilized  eggs  are  laid  in  cells  of  the  reguhir 
size  in  the  wax  comb,  but  unfertilized  eggs  are  laid  only  in 
cells  of  a  larger  size  known  as  drone  cells.  The  fertilized  eggs 
develop  into  females,  even  if  they  are  moved  from  ordinary 
cells  to  drone  cells;  but  the  unfertilized  eggs  produce  males, 
even  if  they  are  transferred  to  cells  of  ordinary  size,  in  which 
case,  however,  they  will  become  small-sized  drones  because 
of  the  limited  amount  of  space  in  which  they  complete  their 
growth.  Fertilized  eggs  developing  in  cells  of  ordinary  lioney- 
comb  size  produce  female  bees  with  imperfectly-developed 
sex  organs,  known  as  workers.  They  are  the  individuals  that 
gather  honey  and  pollen  and  feed  the  young  of  the  colony. 
A  fertilized  egg,  which  produces  a  larva  that  receives  special 
care  and  nourishment  and  develops  in  a  cell  of  unusual  size, 
gives  rise  to  a  queen,  a  fully  developed  female  cai)al)le  of  mat- 
ing and  laying  great  numbers  of  eggs,  but  without  the  struc- 
tural peculiarities  or  instincts  of  workers.  From  these  facts 
it  will  be  clear  that,  in  the  bee,  fertilization  detennines  sex, 
though  environment  (size  of  cell,  food  of  the  lars^a)  may  de- 
termine many  other  characteristics  of  the  individual.  As 
regards  their  origin,  the  female  is  a  zygote  produced  by  the 


258  GENETICS  AND  EUGENICS 

union  of  two  gametes,  the  male  is  derived  from  a  gamete 
developing  by  itself.  So  far  as  chromosome  constitution  is 
concerned,  the  female  is  duplex,  the  male  simplex. 

In  small  Crustacea,  and  rotifers,  the  case  is  slightly  differ- 
ent. The  female  here,  too,  is  duplex  and  the  male  simplex, 
but  the  conditions  of  their  origin  are  less  simple,  for  the 
mother  here  produces  three  different  kinds  of  eggs.  The  first 
kind  never  passes  into  the  simplex  state  of  ordinary  gametes, 
but  retains  the  duplex  number  of  chromosomes,  omits  the 
reducing  cell-division,  and  begins  development  at  once  un- 
fertilized and  duplex.  It  forms  a  female,  like  the  mother  in 
all  respects.  The  other  two  t^^Des  of  eggs  undergo  reduction 
and  pass  into  the  condition  of  gametes,  with  the  simplex  chro- 
mosome number.  They  differ  in  size.  The  smaller-sized  egg 
develops  unfertilized  into  a  male  (simplex)  individual,  which 
forms  simplex  sperm  just  as  the  male  bee  does,  by  omitting 
a  reduction  division  in  spennatogenesis.  The  larger-sized  egg 
(winter  egg)  is  incapable  of  further  development  without  the 
stinmlus  of  fertilization.  When  fertilized,  it  develops  into  a 
female  individual,  since  in  consequence  of  fertilization  it  con- 
tains the  duplex  chromosome  number. 

The  cases  of  bee  and  rotifer  agree  in  this,  that  the  female 
regularly  has  the  duplex  chromosome  condition,  the  male  the 
simplex  condition,  a  difference  completely  parallel  with  that 
between  Oenothera  Lamarckiana  (which  has  fourteen  chro- 
mosomes) and  its  mutant  gigas  (which  has  twenty-eight). 

In  plant  lice  the  difference  between  the  sexes  as  regards 
chromosome  number  is  not  so  great.  Here  the  female  merely 
has  one  or  two  chromosomes  more  than  the  male,  recalling 
the  mutant  Oenothera  lata,  which  has  one  more  chromosome 
than  the  parent  species,  Lamarckiana.  The  male  however 
in  plant  lice  develops  from  an  unfertilized  egg,  partially  re- 
duced in  chromosome  nimiber.  The  female  arises  either  from 
an  egg  unreduced  and  so  with  the  full  duplex  number  of 
chromosomes,  and  which  develops  without  fertilization  into 
a  female,  or  from  a  reduced  egg  (a  true  gamete)  which  has 
been  fertilized  and  thus  brought  back  to  the  duplex  condition. 


SEX  DETERMINATION  259 

If  one  were  inclined  to  be  facetious,  he  might  say  that  in  all 
these  lower  animals,  duplicity  is  synonomous  with  femak-iiess, 
simplicity  with  maleness! 

It  should  be  noted  in  passing  that  among  plants  as  well  as 
among  animals,  an  unfertilized  gamete  may  undergo  multi- 
plication and  growth  while  in  the  simplex,  reduced  condition. 
The  ordinary  fern  plant  is  a  zygote  with  a  duplex  chromo- 
some number.  But  it  produces  reproductive  cells  (spores) 
containing  the  reduced  (simplex)  chromosome  number,  and 
these  after  growing  into  a  small  inconspicuous  little  plant, 
known  as  a  prothallus,  produce  the  functional  gametes  {<^gQ 
and  sperm-cells)  without  further  reduction.  Union  of  these, 
egg  with  sperm,  produces  duplex  zygotes  again,  which 
develop  into  the  ordinary  fern  plant. 

In  many  animals  in  which  males  and  females  alike  arise 
from  fertilized  eggs,  there  occurs  nevertheless  a  difference  in 
chromosome  number  between  males  and  females,  the  female 
always  containing  the  higher  number,  as  in  the  partheno- 
genetic  plant  lice.  One  of  the  best-known  cases  is  that  of 
the  common  squash  bug,  Anasa  tristis,  first  worked  out  by 
E.  B.  Wilson,  but  since  fully  confirmed  by  the  obsers-ations 
of  others.  In  this  animal  the  body-cells  of  the  female  contain 
twenty-two  chromosomes,  those  of  the  male  twenty-one. 
Historically  this  is  a  famous  case,  the  first  one  in  which  the 
mechanism  of  sex  determination  was  definitely  ascertained. 
The  egg,  according  to  Wilson,  always  undergoes  reduction  to 
the  simplex  chromosome  number,  eleven.  But  reduction  in 
the  male  is  less  simple  because  the  male  contains  an  odd 
number  of  chromosomes,  viz.,  twenty-one.  All  the  spenu 
cells  cannot  receive  the  same  number  of  chromosomes  at  the 
reduction  division,  unless  the  odd  chromosome  splits,  but  this 
it  refuses  to  do.  The  division  occurs  into  cells  with  eleven 
chromosomes,  and  those  with  ten.  Both  nietamon^hose  into 
sperm  cells.  The  10-chromosonie  sperm  cells,  if  they  ferti- 
lize an  egg,  cause  it  to  develop  into  a  male,  since  Egg  11  + 
Sperm  10  =  21,  the  number  characteristic  of  the  male.  But 
the   11-chromosome  sperm  fertilizing  an  egg  causes  it  to 


260  GENETICS  AND  EUGENICS 

develop  into  a  female,  since  Egg  11  +  Sperm  11  =  22,  the 
female  number.  The  first  man  to  suggest  a  relation  between 
the  odd  chromosome  and  sex  determination  (McClung)  sup- 
posed of  course  that  the  extra  chromosome  must  go  to  pro- 
duce a  male,  the  more  important  sex,  and  he  called  it  a  male 
sex-determining  chromosome,  but  it  turned  out  otherwise.  The 
extra  chromosome  is  really  a  female  sex  determinant.  When 
a  difference  exists  between  the  sexes  in  chromatin  content, 
it  is  regularly  the  female  that  has  the  larger  supply.  The 
significance  of  this  we  may  inquire  into  further. 

In  some  cases,  several  of  which  are  described  by  Morgan, 
the  number  of  chromosomes  is  found  to  be  the  same  in  both 
sexes,  but  one  of  the  chromosomes  in  the  female  is  regularly 
larger  than  the  corresponding  chromosome  in  the  male.  This 
indicates  that  the  female,  in  this  case  also,  contains  some 
chromosome  element  not  found  in  the  other  sex. 

But  Wilson  and  his  pupils  have  shown  that  in  species  in 
which  the  female  contains  two  X-chromosomes  and  the  male 
one  such  chromosome,  a  new  chromosome  may  appear  in  the 
male,  a  so-called  Y-chromosome,  which  the  female  does  not 
normally  possess.  What  its  precise  function  is  has  not  yet 
been  ascertained. 

Finally,  in  many  animals  no  difference  has  been  detected 
between  the  chromosome  composition  of  the  two  sexes,  but 
this  does  not  preclude  the  existence  of  such  a  difference,  even 
though  it  has  not  yet  been  discovered. 

To  summarize  the  foregoing,  there  are  many  known  facts 
which  support  and  none  which  contradict  the  idea  that  the 
female  has  a  greater  chromatin  content  than  the  male  and, 
either  by  reason  of  this  fact  or  independently  of  it,  has 
greater  anabolic  activity  in  reproduction,  producing  macro- 
gametes,  gametes  stored  with  food.  Micro-gametes,  those 
not  stored  with  food  but  generally  possessed  of  locomotive 
ability,  are  the  distinctive  product  of  males. 

Morgan  (1913)  assumes  that  the  chromatin  element,  which 
occurs  in  the  female  but  not  in  the  male,  is  the  specific  cause 
of  femaleness,  that  is,  of  egg  production,  and  so  speaks  of  the 


SEX  DETERMINATION  2G1 

odd  chromosome  (when  this  occurs)  as  a  sex-cliromosome,  or 
an  X-chromosome.  But  a  moment's  reflection  will  show  (as 
Morgan  himself  once  suggested)  that  quantity  of  such  sub- 
stance may  be  quite  as  influential  as  quality  in  detenniniiig 
sex,  since  by  hypothesis  one  X-chromosome  produces  a  male 
and  two  X-chromosomes  a  female,  in  species  such  as  the 
squash  bug.  The  essential  thing  in  sex  detcnninalion  is 
probably  not  so  much  the  possession  of  some  particular  sort 
of  material  as  the  attainment  of  a  particular  grade  of  ana- 
bolic capacity,  femaleness  implying  a  higher  grade  than  niale- 
ness,  since  in  the  former  condition  macro-gametes  are  pro- 
duced, whereas  in  the  latter  micro-gametes  are  produced. 

That  maleness  and  femaleness  are  only  different  grades  of 
reproductive  capacity  is  indicated  by  a  study  of  organisms 
in  which  the  two  functions  are  combined.  In  many  of  the 
lower  animals  and  in  most  of  the  higher  plants,  the  same  in- 
dividual is  capable  of  producing  both  macro-gametes  and 
micro-gametes.  Sometimes  these  are  produced  simultane- 
ously but  in  separate  gonads,  as  in  flatwomis  and  leeches 
among  animals,  and  in  "  perfect  "  flowering  plants.  Such 
parents  are  true  and  simultaneous  hermaphrodites.  Some- 
times the  individual  may  function  at  first  as  a  male  and  later 
as  a  female,  a  condition  known  as  successive  hennaphro- 
ditism.  This  is  found  in  certain  worms  and  moUusks  and  in 
the  prothallia  of  certain  ferns  and  mosses.  This  condition  is 
also  approached  in  flowering  plants  such  as  cucinnbers, 
melons,  and  squashes,  which  at  first  produce  only  male  blos- 
soms but  later  produce  those  of  both  sexes.  In  other  cases 
the  individual  may  function  chiefly  as  of  one  sex  but  partially 
as  of  the  other  sex.  This  condition  is  found  in  polygamo- 
dioecious  plants  and  exceptionally  in  such  animals  as  cray- 
fish, mollusks,  worms,  and  even  frogs  and  fishes,  which,  in  a 
particular  part  of  an  ovary  may  develop  sperms,  or  in  a 
particular  part  of  a  testis  may  develop  eggs. 

Such  facts  as  these  indicate  that  maleness  and  fenudeness 
are  merely  different  grades  of  one  and  the  same  f onn  of  repro- 
ductive activity.    This  is  not  inconsistent  with  their  behavior 


262  GENETICS  AND  EUGENICS 


—  \V .•  V  : 


as  Mendelian  alternatives  in  heredity,  for  in  color  inheritance 
different  grades  of  pigmentation,  of  spotting,  etc.,  frequently 
)3ehave  as  Mejpidelian  allelomorphs.  So  probably  different 
degrees  of  sexual  distinctness  behave  in  heredity,  for  in  the 
plant,  Lychnis,  Shull  has  shown  that  femaleness  is  allelo- 
morphic  not  only  with  maleness  but  also  with  hermaphro- 
ditism, the  three  conditions  being  triple  allelomorphs.  A 
similar  interpretation  may  perhaps  be  given  to  conditions 
found  in  certain  mosses  as  discussed  by  Collins. 


!•. 


PrK'ate  Property  of 

^'  P.  rvlETCALF 


PART  II 

EUGENICS 


<0 


CHAPTER  XXX 


HUMAN  CROSSES 


Mi^^NKiND  consists  of  a  single  species;  at  least  no  races  exist 
so  c^istinct  that  when  they  are  crossed  sterile  progeny  are  pro- 
duced. The  widest  possible  human  crosses  are  comparable 
with  the  crossing  of  geographical  varieties  of  a  wild  species 
of  animal,  or  with  the  crossing  of  distinct  breeds  of  domesti- 
cated animals.  The  race  horse  and  the  draft  horse  differ  as 
muph  in  bodily  conformation  and  temperament  as  do  the 
Dio^^t  diverse  races  of  mankind. 

0/fspring  produced  by  crossing  such  races  do  not  lack  in 
vigor,  size  or  reproductive  capacity.  But  these  are  not  the 
only  qualities  w^hich  we  desire  either  our  horses  or  our  citi- 
zens ^  to  possess.  It  is  a  particular  combination  of  qualities 
whic:}!  makes  a  race  horse  useful,  and  a  different  combination 
whicl|i  makes  a  draft  horse  useful.  Crossing  the  two  will 
produ^ce  neither  one  type  nor  the  other.  The  progeny  will 
be  use.jess  as  race  horses  and  they  will  not  make  good  draft 
horses.  A  second  generation  of  offspring  will  be  more  vari- 
able but  will  rarely  approach  the  specialized  type  of  either 
the  race  hvr)rse  or  the  draft  horse,  and  will  be  too  heterogene- 
ous in  chan^cter  to  serve  any  single  purpose  well.  For  such 
reasons  as  Lhese,  pure  breeds  of  domesticated  animals  are 
rarely  crossed  unless  a  new  type  of  animal  is  desired  to  meet 
special  needs  i^^^nd  conditions.  Even  then  many  animals  of 
small  value  musi'^  be  produced  and  discarded  and  this  process 
must  be  continue!  i  for  generations  before  the  new  type  can 
be  established.  For  such  reasons  wide  racial  crosses  among 
men  seem  on  the  whoJe  undesirable.  There  is  no  question 
about  the  physical  vigeor  of  the  offspring,  provided  the 
parents  are  free  from  disea^se.  The  statement  is  often  made 
that  mixed  races  are  feeble,  btut  if  this  is  ever  true  it  is  not 
because  they  are  mixed,  but  beca-use  the  specimens  that  mix 

m       \ 


we  GENETICS  AND  EUGENICS 

are  feeble.  Mating  out  of  the  race,  when  mates  within  he 
race  are  available,  is  prima  facie  evidence  that  the  individial 
so  mating  is  a  social  outcast.  It  is  not  surprising  that  he 
progeny  of  such  individuals  are  sometimes  feeble.  If  he 
parents  were  diseased,  licentious,  or  feeble-minded,  i^  is 
natural  that  the  children  should  be  of  like  character. 

Of  course  not  all  racial  crossing  implies  such  conditiois. 
Frequently  Europeans,  when  pioneers  in  a  new  country  iud 
without  mates  of  their  own  race,  have  married  native  wonen. 
Such  men  have  not  always  been  social  outcasts;   frequertly 
they  have  been  men  of  great  energy,  ability,  and  courige 
both  physical  and  moral,  and  free  from  disease.     When  in 
such  cases,  the  mothers  belonged  to  a  race  with  capacity  for 
civilization,  the  results  have  been  good.    Examples  ma^  be 
found  among  the  Indian  citizens  of  our  southwest  states. 
But  human  racial  crossing  in  general  is  a  risky  experiment, 
because  it  interferes  with  social  inheritance,  which  after  all 
is  the  chief  asset  of  civilization.    Physically  and  also  intel- 
lectually, according  to  Professor  Osborn,  we  are  no  whit 
superior  to  the  men  of  twenty-five  thousand  years  ago.    All 
the  advantage  which  we  have  over  them  lies  in  the  accumu- 
lated experience  of  the  human  race  since  then. 

All  this  we  as  individuals  learn  from  our  mothers  and 
fathers,  or  in  the  schools,  the  churches,  the  markets,  or  the 
courts  of  justice.  Wide  racial  crosses  unsettle  the  founda- 
tions of  these  agencies  of  enlightenment.  At  times  it  is 
necessary  that  some  of  these  agencies  be  distur'bed  in  order 
that  we  may  lay  their  foundations  deeper  and  broader,  but 
racial  crossing  leads  rather  toward  the  discarding  of  all 
foundations  of  civilization  than  to  improvin.g  them. 

Such  crosses,  therefore,  as  of  Europear^s  with  Asiatics  or 
Africans  can  not  be  recommended  as  age^acies  for  the  improve- 
ment of  the  human  race.  Physically  T^uropeans  on  one  hand 
and  Asiatics  or  Africans  on  the  oth-^^r,  are  suflSciently  diversi- 
fied among  themselves  to  allow  'the  maximum  benefit  from 
intercrossing,  without  resort^'^ng  to  crosses  with  a  distinct 
branch  of  the  human  fam^ily.     Socially  the  effects  of  such 


\ 


i 


HUMAN  CROSSES  2ii7 

crosses  on  a  large  scale  are  too  disturbing  to  be  recommended. 
This  country  has  seen  a  suflSciently  extensive  experiment  of 
that  sort  in  its  southern  states,  the  outcome  of  which  we 
shall  not  know  fully  for  several  generations  yet.  It  is  desir- 
able that  each  nation  should  have  the  fullest  intercourse  with 
every  other  in  commerce  and  in  the  exchange  of  ideas.  This  is 
mutually  beneficial  to  all,  but  the  obliteration  of  all  racial 
differences  within  the  human  family  is  not  to  be  expected  or 
desired. 

What  has  been  said  thus  far  refers  only  to  crosses  between 
the  widely  separated  branches  of  the  human  family  and  even 
as  regards  such  cases  may  be  accepted  with  reservation,  since 
there  is  room  for  a  difference  of  opinion  concerning  such 
matters,  which  are  not  primarily  biological,  but  sociological. 

What  opinion  one  holds  will  also  depend  upon  his  point  of 
view.  From  the  viewpoint  of  a  superior  race  there  is  nothing 
to  be  gained  by  crossing  with  an  inferior  race.  From  the 
viewpoint  of  the  inferior  race  also  the  cross  is  undesirable  if 
the  two  races  live  side  by  side,  because  each  race  w  ill  despise 
individuals  of  mixed  race  and  this  will  lead  to  endless  friction. 
About  the  only  conditions  under  which  a  racial  cross  of  this 
sort  could  be  fairly  tested  would  be  those  under  which  Pit- 
cairn  Island  was  populated.  Here  more  than  a  century  ago 
a  few  English  sailors  and  a  few  Polynesian  women  founded  a 
population  still  in  existence  and  flourishing.  Neither  pure 
race  was  present  to  create  social  distinctions  or  racial  anti- 
pathy. The  story  of  this  hybrid  human  race  is  a  romantic  one. 

In  the  year  1788  the  Englishman,  John  Bligh,  who  as 
sailing  master  had  been  round  the  world  with  Captain  Cook 
on  his  second  voyage,  was  commissioned  by  the  British 
Government  to  go  to  Tahiti,  secure  plants  of  the  bread-fruit 
tree  and  introduce  them  into  the  West  Indies.  To  this  end 
he  was  given  command  of  the  ship  Bounty.  Bligh  proved  a 
harsh  and  oppressive  captain,  and  on  his  way  from  Tahiti  to 
Jamaica  the  crew  mutinied.  They  put  the  captain  with 
eighteen  of  his  crew  into  the  ship's  launch  and  themselves 
turned  back  to  Tahiti.    The  captain  and  his  companions  after 


1268  /        GENETICS  AND  EUGENICS 

three  months  of  hardship  all  reached  land  (Timor,  three 
thousand  six  hundred  miles  from  where  they  started)  safely, 
and  were  taken  back  to  England.  The  British  Government 
sent  out  a  warship  to  punish  the  mutineers  and  part  of  them 
were  captured  on  Tahiti.  But  their  leader  and  nine  other 
sailors  had  already  escaped  to  Pitcairn  Island  in  company 
with  eighteen  natives,  six  men  and  twelve  women.  Their 
place  of  refuge  remained  a  secret  for  twenty  years,  when  it 
was  accidentally  discovered  by  an  American  sealing  ship 
which  visited  the  island  in  1808.  Pitcairn  Island  is  the 
southernmost  island  of  the  Low  Archipelago  in  latitude  25° 
S.  and  longitude  180°  W.  It  is  about  two  miles  long  and  one 
mile  wide,  and  consists  of  a  mountain  surrounded  by  coral 
reefs.  For  ten  years  after  the  landing  of  the  refugees,  dis- 
order and  lawlessness  prevailed.  In  1808  the  sole  survivors 
were  one  Englishman  by  the  name  of  John  Adams  (formerly 
Alexander  Smith),  eight  or  nine  women,  and  several  children. 
It  is  related  that  the  elements  of  disorder  being  removed 
Adams  instilled  ideas  of  morality  and  religion  into  the  others, 
with  the  result  that  the  settlement  prospered.  In  1815  when 
the  ship  Britain  visited  the  island,  the  captain  was  impressed 
with  the  peace  and  good  order  prevailing.  In  1839  the  island 
became  a  British  dependency.  In  1855  the  number  of  in- 
habitants had  increased  to  two  hundred  and  the  island  was 
becoming  too  small  for  them.  They  therefore  petitioned  the 
British  government  to  be  removed  to  Norfolk  Island,  which 
was  done  the  following  year.  Since  then  some  of  them  have 
returned  to  Pitcairn  Island  whose  present  population  is  about 
one  hundred  and  twenty -five.  The  population  of  Norfolk 
Island  in  1901  was  eight  hundred  and  seventy,  mostly 
descendants  of  the  Pitcairn  Islanders. 

Here  then  on  these  two  islands  is  a  race  of  probably  one 
thousand  persons  at  the  present  time,  originated  more  than 
a  century  ago  by  a  cross  between  English  men  and  women  of 
Tahiti.  The  experiment  has  gone  far  beyond  the  Fi  genera- 
tion and  would  afford  unique  material  for  a  study  of  the 
effects  of  race-crosses  uncomplicated  by  race-antipathies.    So 


HUMAN  CROSSES  269 

far  as  present  information  goes  the  results  have  been  excellent 
both  biologically  and  sociologically.  It  is  to  be  hoped  that 
some  student  of  eugenics  will  give  the  case  careful  and 
critical  study. 

Another  successful  experiment  in  human  racial  crossing  has 
been  recently  studied  and  described  by  a  German,  Fischer,* 
who  chronicles  the  origin  of  a  tribe  in  Genu  an  Southwest 
Africa  of  mixed  Boer  and  Hottentot  blood.  This  arose  from 
the  intermarriage  with  native  Hottentots  of  a  few  Boers  dis- 
satisfied with  British  rule  in  South  Africa,  who  penetratecl 
far  northward  among  hostile  tribes,  and  were  thus  forced  to 
combine  with  each  other  against  a  common  enemy.  IMieir 
descendants,  intermarrying,  formed  a  distinct  cultural  grouj) 
entirely  surrounded  by  pure  native  stocks  and  wholly  isolated 
from  contact  with  Europeans.  Pride  in  their  ancestry  and 
cultural  inheritance  held  them  together  and  prevented  mix- 
ing with  neighboring  tribes.  After  this  had  gone  on  for 
several  generations  they  came  within  the  German  zone  of 
colonial  influence  (again  British  at  present  under  the  fortune 
of  war).  Very  likely  the  group  as  such  will  presently  dis- 
appear, but  the  experiment  has  progressed  far  enough  to 
show  that  under  conditions  which  do  not  interfere  with 
cultural  inheritance  crossing  of  racial  stocks  as  widely  sepa- 
rated as  Europeans  and  Africans  has  no  evil  consequences, 
but  produces  a  vigorous,  sound  race.  Fischer  finds  evidence 
of  Mendelian  inheritance  of  physical  characters  among  these 
people,  but  critically  examined,  this  evidence  is  substantially 
like  that  available  from  other  sources.  Some  characters, 
such  as  hair  and  eye-colors  show  fairly  good  segregation.  As 
regards  skin-color,  proportions  of  the  skeleton,  features,  etc., 
the  hybrids  are  intermediate  between  the  parent  races,  but 
more  variable.  It  is  probable  that  intelligence  and  other 
psychic  traits  are  inherited  in  this  way. 

Racial  crosses,  if  so  conducted  as  not  to  interfere  with 
social  inheritance,  may  be  expected  to  produce  on  the  whole 
intermediates  as  regards  physical  and  psychic  characters. 

1  "  Die  Rhehobothen  Bastarden,"  1911. 


270 


GENETICS  AND  EUGENICS 


This  seems  to  have  been  the  result  in  Central  and  South 
America  and  in  the  West  Indies,  where  racial  crossing  has 
taken  place  to  a  very  great  extent.  A  similar  outcome  seems 
likely  to  occur  in  Africa,  as  that  continent  is  further  overrun 
by  European  races.  The  leading  racial  stocks  of  Asia  seem 
at  the  present  moment  to  have  such  physical,  mental,  and 
cultural  vigor  that  they  are  not  likely  to  amalgamate  with 
European  races. 


i 


>  ( 


CHAPTER  XXXI 

PHYSICAL  AND  MENTAL  INHERITANCE  IN  MAN 

The  same  laws  govern  inheritance  in  man  as  in  other  animals 
and  in  plants,  but  our  knowledge  of  human  heredity  is  k-ss 
accurate  than  that  of  animals  and  plants,  because  we  are  in 
the  human  field  debarred  from  experiment,  llie  best  we 
can  do  is  to  observe  and  compare  the  traits  of  individuals  in 
successive  generations  and  thus  to  ascertain  with  what  known 
laws  of  heredity  these  cases  best  agree.  For  the  discovery  of 
new  laws  of  heredity,  human  data  can  have  little  value  be- 
cause of  our  inability  to  experiment.  Nevertheless  the  inter- 
est in  human  heredity  is  so  general  and  the  num})er  of 
competent  observers  so  large,  including  as  it  does  a  great 
many  physicians  and  other  men  of  science,  that  we  may  look 
forward  to  a  very  complete  cataloguing  of  human  heredity 
as  fast  as  general  categories  of  inheritance  phenomena  are 
established  by  the  experimental  study  of  other  organisms. 
Already  we  have  in  hand  a  great  amount  of  material  bearing 
on  human  heredity,  gathered  chiefly  by  medical  men,  much 
of  it  within  the  last  fifteen  years.  A  considerable  part  of  this 
is  unreliable  because  of  the  careless  or  biased  way  in  which  it 
has  been  gathered,  or  the  uncritical  treatment  which  it  has 
received  in  publication.  But  still  there  remains  a  consider- 
able body  of  valuable  information,  which  shows  that  man  is 
subject  to  heredity  in  every  aspect  of  his  physical  and 
mental  make-up. 

Two  comprehensive  attempts  have  been  made  to  gather 
and  analyze  data  concerning  human  inheritance,  one  in  Eng- 
land at  the  Eugenics  Laboratory  of  the  LTniversity  of  Loiulon, 
founded  by  Galton  and  presided  over  by  Karl  Pearson,  the 
other  and  more  recent  one  at  the  Eugenics  Record  Oflice, 
Cold  Spring  Harbor,  New  York,  directed  by  Dr.  C.  R. 
Davenport.    Pearson's  data  are  recorded  in  the  "  Treasury 

271 


27^2 


GENETICS  AND  EUGENICS 


TABLE   33 

Inherited  Characters  in  Man 

1 .   Blending 

General  body  size,  stature,  weight,  skin-color,  hair-form  (in  cross-section,  corre- 
lated with  straightness,  curliness,  etc.)  shape  of  head  and  proportions  of  its  parts 
(feat  ares). 

2.   Mendelian 
Dominant 


Skin  and  hair  ' 


Eyes 


Dark. 

Spotted  with  white. 

Tylosis  and  ichthyosis  (thickened 
or  scaly  skin). 

Epidermolysis  (excessive  forma- 
tion of  blisters). 

Hair  beaded  (diameter  not  imi- 
form). 

Front    of    iris    pigmented    (eye 

black,  brown,  etc.). 
Hereditary  cataract. 
Night  blindness  (when  not  sex 

limited). 
Normal. 


Recessive 

Blonde   or   albino    (probably 

multiple  allelomorphs). 
Uniformly  colored. 
Normal  skin. 

Normal  skin. 

Normal  hair. 


Only  back  of  iris  pigmented 

(eye  blue). 
Normal. 
Normal. 

Pigmentary    degeneration    of 
retina. 


r-\ 


Skeleton 


Kidneys 


K 


Brachydactyly  (short  digits  and 

limbs). 
Polydactyly  (extra  digits). 
Syndactyly    (fused,    webbed,  or 

reduced  number  of  digits). 
Symphalangy    (fused    joints    of 

digits,  stiff  digits). 
Exostoses  (abnormal  outgrowths 

of  long  bones). 
^  Hereditary  fragility  of  bones. 

Diabetes  insipidus,  (excessive  pro- 
duction of  urine). 
Normal. 


Normal. 

Normal. 
Normal. 

Normal. 


Normal. 


Normal. 


Alkaptonuria  (urine  black  on 
oxidation). 


Nervous      j  Huntington's  chorea. 
System       \  Normal. 


Normal. 

Hereditary  feeble-mindedness. 


HUIVIAN  INHERITANCE  <27f^ 

3.   Mendelian  and  Sex-Linked 
(Appearing  in  males  when  simplex,  but  in  females  only  when  duplex.) 
Dominant  Recessive 

Normal.  Gower's  muscuhir  atrophy. 

Normal.  Haemopliiliu  (liltM-ilin;?).  ' 

Normal.  Color  blindness  (inability  to 

distinguish  red  from  ^n't'n). 
Normal.  Night  blindn<'ss  (inability  to 

see  in  faint  light j, 

4.    Probably  Mendelian  but  Dominance  Uncertain  or  Imperfect 

Defective  hair  and  teeth  or  teeth  alone,  extra  teeth,  a  double  set  of  permanent 
teeth,  hare-lip,  cryptorchism  and  hypospadias  (imperfectly  developed  male  organs), 
tendency  to  produce  twins  (in  some  families  determined  by  the  father,  in  others  by 
the  mother),  left-handedness,  otosclerosis  (hardness  of  hearing  owing  to  thickened 
tympanum). 

5.   Subject  to  Heredity,  but  to  what  Extent  or  how  Inherited  Uncertain 

General  mental  ability,  memory,  temperament,  musical  ability,  literary  al)ility, 
artistic  ability,  mathematical  ability,  mechanical  ability,  congenital  deafness,  lia- 
bility to  abdominal  hernia,  cretinism  (due  to  defective  or  diseased  thjToids),  defec- 
tive heart,  some  forms  of  epilepsy  and  insanity,  longevity. 

of  Human  Inheritance"  (1909).  The  data  collected  by  the 
Eugenics  Record  Office  have  been  published  in  part  in  a 
series  of  bulletins  and  monographs  which  is  being  rapidly 
extended. 

We  may  provisionally  distinguish  inherited  human  traits 
as  (1)  blending  (probably  involving  multiple  factors);  (2) 
clearly  Mendelian  (involving  a  single  genetic  factor);  (3) 
Mendelian  and  sex-linked;  (4)  probably  Mendelian  but  with 
dominance  imperfect  or  uncertain,  and  (5)  hereditary,  but 
to  what  extent  or  how,  uncertain. 

The  grounds  on  which  a  category  of  blending  characters 
may  be  based  have  already  been  discussed.  If  they  are  valid 
for  animals  and  plants,  they  are  also  valid  for  man.  Here 
belong  characters  which  show  intermediate  inlieritance  in  Fi 
and  also  in  F2,  but  with  greater  variability  in  Fj  than  in  F,. 
Size  and  stature  are  good  examples.  The  greater  variability 
of  F2  shows  that  the  blending  was  not  perfect  in  F,  and  that 
multiple  factors  are  probably  involved.  Iiidieatioiis  of 
segregation  more  or  less  complete  were  observed  by  Daven- 


274  GENETICS  AND  EUGENICS 

port  in  his  studies  of  skin-color  and  hair-form  inheritance 
in  negro-white  crosses,  which  supports  the  idea  that  mul- 
tiple factors  are  involved,  or  one  or  more  chief  factors  as- 
sociated with  modifying  factors.  The  well  known  lack  of 
correlation  between  skin-color  and  hair-form  in  mulattoes  of 
the  F2  or  later  generations  certainly  indicates  the  existence 
of  independent  factors  affecting  these  characters. 

As  regards  shape  of  the  head,  anthropologists  have  long 
distinguished  between  long-headed  and  round-headed  races 
or  types  within  mixed  races.  These  may  be  convenient  terms 
for  purposes  of  classification,  but  it  by  no  means  follows  that 
the  t}^es  are  alternative  in  heredity.  Without  positive  evi- 
dence to  the  contrary,  it  is  safe  to  assume  from  what  we  know 
of  skull  shape  in  animals  and  in  negro-white  crosses  that 
skull  shape  is  in  all  cases  blending  (multiple  factorial)  in  in- 
heritance. Salaman  (1911)  himself  an  English  Jew,  has  de- 
scribed the  Jewish  tj^^e  of  countenance  as  recessive  to  the 
Anglo-Saxon  type  in  mixed  marriages  in  England  on  classi- 
fications of  the  offspring  as  of  Jewish  or  Gentile  type,  made 
for  him  by  Jews,  but  the  evidence  is  far  from  satisfactory  and 
not  based  on  any  clearly  defined  differences.  If  measurable 
characters  were  considered,  it  is  probable  the  inheritance 
would  be  found  to  be  blending,  and  the  classification  adopted 
in  his  tables  to  have  been  based  on  blending  in  many  char- 
acters rather  than  on  simple  segregation  in  any  one. 

It  is  to  be  noted  that  in  man,  as  in  wild  species  of  animals 
and  plants,  characters  which  hlend  in  heredity  are  in  no  case 
abnormal  or  monstrous  conditions,  but  are  such  as  distin- 
guish one  member  of  a  perfectly  normal  population  from 
another. 

The  case  is  very  different  when  we  come  to  the  category  of 
simple  Mendelian  characters,  whether  or  not  sex-linked. 
Here  a  great  majority  of  the  characters  listed  refer  to  abnor- 
malities or  monstrosities.  As  regards  variation  in  the  color  of 
hair,  skin  and  eyes,  we  have,  in  these,  recessive  or  loss  varia- 
tions, similar  to  those  of  other  mammals,  producing  a  graded 
series  of  probable  allelomorphs  ranging  from  black  to  albino. 


HUMAN  INHERITANCE  07", 


z  ( o 


Retrogressive  variation  of  eye  pigmentation  leads  from 
''heavily  pigmented  iris  (back  and  front)"  through  inoiv 
faintly  pigmented  conditions  to  *'iris  pigmented  only  be- 
hind," the  ultimate  recessive,  blue.  Spotting  with  while, 
affecting  skin  and  hair  pigmentation,  or  affecliug  only  Ihc 
pigmentation  of  the  iris  (Bond,  1912)  are  unit-character 
variations  completely  parallel  with  those  of  rodents.  Nearly 
all  other  known  Mendelizing  characters  in  man  are  more  or 
less  pathological.  They  include  a  variety  of  h(Teditary  mal- 
formations or '* diseases"  affecting  skin,  eye,  skeleton,  kid- 
neys or  nervous  system.    (See  Table  33.) 

Many  characters  (mostly  loss  variations)  are  probably 
Mendelian  in  inheritance,  but  not  enough  is  known  concern- 
ing their  behavior  to  permit  of  a  positive  statement  in  the 
matter.     (See  Table  33,  4.) 

In  Section  5  of  Table  33  are  included  many  important 
characters  known  to  be  to  some  extent  hereditarv,  but  in 
accordance  with  what  law  is  still  uncertain.  Especially  im- 
portant are  such  characters  as  general  mental  ability,  mental 
capacity  in  special  directions,  hereditary  epilepsy  and  in- 
sanity, and  longevity.  It  would  be  a  mistake  to  cover  up 
our  present  ignorance  concerning  the  inheritance  of  these 
characters  by  classifying  them  either  as  unifactorial  or  as 
multifactorial.  We  shall  presently  examine  into  the  evidence 
that  the  more  important  of  these  are  inherited. 

Hair-form.  This  character  has  been  studied  by  Dr.  and 
Mrs.  Davenport,  whose  findings  may  be  briefly  sunnnarized. 
Hair  having  a  circular  cross-section  is  straight.  But  if  the 
hair  is  elliptical  in  cross-section,  it  has  a  tendency  to  become 
curly.  Grades  of  departure  from  the  straight  condition  are 
formed  with  increase  in  flattening  of  the  hair  in  cross-section 
as  follows:  (1)  straight,  (2)  wavy,  (3)  curly,  (4)  kinky  (Afri- 
cans). Crosses  produce  intermediates  or  show  imperfect 
dominance  of  curliness,  with  segregation  more  or  less  com- 
plete in  later  generations. 

Hair  and  skin-color.  Hair-color  is  in  general  correlateii 
with  skin-color,  the  darkest  shades  of  hair-color  l)eing  found 


276  GENETICS  AND  EUGENICS 

only  in  persons  with  dark  skin.  Whole  races  of  mankind 
have  only  black  hair  and  dark  skin  (known  as  ''black,  brown, 
red  or  yellow").  A  dark  skin  is  an  adaptation  to  life  in  a 
tropical  country  or  one  having  much  intense  sunlight.  Fair- 
skinned  races  are  unable  to  endure  life  in  the  tropics  unless 
the  body  is  protected  from  the  direct  rays  of  the  sun.  Dark- 
skinned  races,  however,  have  a  natural  protection  against 
the  effects  of  direct  sunlight.  From  an  evolutionary  stand- 
point the  white  races  are  possibly  retrogressive  variations, 
*'loss"  variations.  In  a  population  of  Europeans,  the  darker 
shades  of  hair  and  skin-color  are  either  completely  or  incom- 
pletely dominant.  It  is  not  at  all  uncommon  to  find  a 
mixture  of  dark-haired  and  light-haired  children  in  the 
same  family,  provided  one  or  both  parents  are  dark-haired, 
but  when  both  parents  are  light-haired,  the  children  are  all 
light-haired.  This  result  shows  that  the  lighter  shades  of 
hair-color  are  recessive  in  relation  to  the  darker  shades.  An 
exact  estimate  is  often  difiScult  to  make  because  persons  with 
light  hair  in  childhood  often  have  much  darker  hair  when 
adult,  and  further,  the  hair  may  later  become  gray  or  even 
white,  which  makes  direct  comparison  with  the  hair  of 
younger  persons  impossible. 

Extremely  pale  conditions  of  hair,  skin  and  eye  pigmenta- 
tion are  known  as  albinism  and  occur  in  all  races,  even  in 
negroes  and  American  Indians.  Albinism  is  clearly  a  reces- 
sive character  in  relation  to  normal  pigmentation.  The  vari- 
ous shades  of  blonds  probably  correspond  physiologically 
and  as  regards  inheritance  with  the  graded  series  of  albino 
allelomorphs  found  in  guinea-pigs.  Each  darker  shade  is 
dominant  to  the  lighter  shades,  any  two  in  the  entire  series 
being  allelomorphs  of  each  other.  This  is  known  to  be  the 
case  in  rodents  and  probably  holds  for  European  races  of 
mankind.  In  other  races  of  mankind  blond  variations  are 
rare,  even  more  so  than  extreme  albinism.  Here  again  we 
have  a  condition  parallel  with  that  found  in  most  rodents,  in 
which  the  albino  variation  is  known,  but  not  other  members 
of  the  graded  series  of  retrogressive  allelomorphs. 


HUMAN  INHERITANCE  277 

In  a  cross  between  a  negro  and  a  white  person,  children  are 
produced  of  an  intermediate,  but  frequently  varial)le  skin- 
color,  and  are  known  as  mulattoes.  Mulattoes  mating  ijifer  se 
produce  an  F2  generation  of  highly  variable  skin-color  but 
rarely  pure  white.  Davenport  has  concluded  that  two  inde- 
pendent Mendelian  factors  affecting  skin-color  are  involved. 
This  explanation  would  lead  us  to  expect  one  in  sixteen  of  the 
F2  mulatto  offspring  to  have  skin  as  white  as  a  Euroixan, 
even  though  his  negro  ancestry  might  show  in  other  charac- 
teristics, such  as  curly  hair,  broad  nose,  thick  lips,  etc.  It  is 
difficult  to  get  any  wholly  satisfactory  evidence  either  for  or 
against  this  explanation.  That  published  by  Davenport  can 
scarcely  be  considered  conclusive,  for  the  data  studied  are 
derived  from  a  population  in  which  illegitimacy,  by  Daven- 
port's own  statement,  is  as  high  as  72  per  cent.  On  the 
whole,  it  seems  probable  that  segregation  of  skin  pigmenta- 
tion in  mulattoes  is  either  incomplete  or  rarely  complete,  be- 
cause multiple  or  modifying  factors  are  involved. 

A  clearly  and  sharply  defined  Mendelian  factor  which  in- 
volves spotting  with  white  occurs  in  many  human  families, 
as  in  domesticated  animals.  In  some  families  a  lock  of  white 
hair  (usually  above  the  middle  of  the  forehead,  or  on  top  of 
the  head)  is  inherited  as  a  Mendelian  dominant  (transmitted 
only  through  affected  individuals).  Irregular  spotting  of  the 
body  with  unpigmented  areas  has  been  shown  to  be  heredi- 
tary as  a  dominant  character  in  a  family  of  Louisiana  n^»ror.; ®  L  A c> 
(exhibited  in  Europe  and  America),  and  a  similar  variation 
is  inherited  in  the  same  way  in  a  white  family  in  Minnesota, 
one  or  more  of  whom  have  studied  at  the  University  of 
Minnesota. 


s 


i 


CHAPTER  XXXIl 

HEREDITY  OF  GENERAL  MENTAL  ABILITY,  INSANITY, 
EPILEPSY,  AND  FEEBLE-MINDEDNESS 

One  of  the  first  investigations  carried  on  in  the  la])oralory 
of  Pearson  related  to  the  inheritance  of  abiUty  as  indicated 
by  the  "  class  lists  "  (rank  lists)  of  Oxford.  The  investigation 
of  the  relative  rank  of  two  thousand  five  hundred  pairs  of 
fathers  and  sons  showed  that  a  distinct  correlation  exists 
between  them.  If  the  father  took  high  rank  the  son  also 
ranked  high,  and  vice  versa,  in  a  considerable  percentage  of 
cases.  Expressed  numerically  the  correlation  in  the  Oxford 
lists  was  found  to  be  .31  where  1.00  would  express  exact 
agreement  in  rank  and  0  would  express  only  chance  agree- 
ment. Between  four  thousand  two  hundred  brothers  the 
agreement  was  closer  still,  viz.,  .40.  Closer  resemblance  was 
indeed  to  be  expected,  since  in  this  case  the  mothers  as  well 
as  the  male  ancestors  were  the  same.  The  conclusion  reached 
is  that  mental  capacity,  as  indicated  by  rank  attained  at  the 
University,  is  inherited;  that  the  proverb  "like  father,  like 
son  "  applies  in  the  long  run  to  scholarship,  as  well  as  to 
physique.  This  is  a  conclusion  which  every  experienced 
teacher  would  have  anticipated.  It  is  interesting  to  find  that 
it  has  full  statistical  warrant. 

But  the  further  question  arises  whether  success  in  study 
has  any  relation  to  success  in  life  outside  of  schools.  Of  this 
question  an  investigation  was  made  in  Pearson's  laboratory. 
Rank  in  the  Oxford  B.  A.  examinations  was  compared  with 
subsequent  rank  in  the  professions,  the  Church  and  the  Law. 
The  measure  of  success  in  the  Church  was  taken  to  be  the 
holding  of  a  high  office  in  the  Church  or  of  a  first-class 
scholastic  appointment.  It  was  found  that  the  higher  the 
classification  of  a  man  at  the  Oxford  examinations,   the 

• 
279 


280  GENETICS  AND  EUGENICS 

brighter  were  his  prospects  of  attaining  distinction  in  the 
Church. 

Rank  in  Percentage 

Oxford  Examinations  Distinguished 

First  class 68 

Second  "    37 

Third    "    32 

Fourth  "    29 

Pass  degree 21 

No  degree 9 

Of  those  who  attained  a  first-class  degree,  68  per  cent  ob- 
tained ofl&cial  distinction,  etc. 

The  results  of  the  investigation  as  regards  lawyers  were 
found  to  be  very  similar.  The  measure  of  success  here  was 
taken  to  be  the  holding  of  public  office  under  the  government. 

Of  the  first  class  men,  46  %  were  so  distinguished. 

«     "    second"         "      33%. 

"     «    third     "         "     22%. 

"  "  fourth  "  "  20%. 
Pass  degree  men,  16%. 

No  degree  men,  15  %. 

The  general  conclusion  reached  is  that  the  "  promise  of 
youth  "  as  indicated  by  scholarship  is  in  general  justified  by 
the  "  performance  of  manhood  "  in  the  professions.  The 
objection  might  be  offered  that  appointments  in  church  and 
state  may  be  influenced  by  a  man's  university  rank,  but  this 
is  offset  by  results  obtained  in  America,  where  this  is  cer- 
tainly not  true. 

Insanity.  Considerable  work  has  been  done  in  Pearson's 
laboratory  in  the  study  of  the  heritability  of  insanity.  David 
Heron  made  a  study  of  the  inheritance  of  insanity  as  indi- 
cated by  three  hundred  and  thirty-one  family  histories  col- 
lected during  a  period  of  thirty  years  by  the  superintendent 
of  an  asylum  patronized  by  middle-class  people  of  Perth, 
Scotland.    See  Table  34. 

If  insanity  is  treated  as  due  to  one  and  the  same  thing  in 
all  cases,  it  is  obvious  that  the  inheritance  is  not  Mendelian; 
i.  e.,  insanity  does  not  behave  as  a  simple  Mendelian  unit- 
character,  either  dominant  or  recessive.    But  that  insanity 


INSANITY  C2.S1 

is  in  some  way  inherited  is  obvious,  for  it  occurs  much 
oftener  in  these  families  than  in  the  general  i)oi)ulati()ii, 
where  it  is  between  1  and  2  per  cent.  But  in  these  fanu'lic.s 
21  per  cent  of  the  offspring  of  sane  parents  are  insane,  and 
a  still  higher  percentage  of  the  offspring  of  insane  parents 
are  insane. 

The  correlation  coeflBcient  used  as  a  measure  of  the 
strength  of  the  inheritance  of  insanity  lies  ])etvveen  .52  and 
.62.  For  comparison  it  may  be  said  that  the  correlation  co- 
eflScient  between  parent  and  child  in  the  case  of  pulmonary 

TABLE  34 
Data  on  Inheritance  of  Insanity  (Heron) 

Children 

Parents                                            Insane  Sanr  %  Insane 

Both  sane 314  1179  21 

One  insane 93  299  24 

Both  insane 4  4  50 

tuberculosis  was  found  by  Pearson  to  be  about  .50;  for  deaf- 
mutism  1  it  was  found  to  be  .54;  for  stature  .50;  for  intelli- 
gence between  .49  and  .58. 

Heron  concludes  that  insanity  on  the  whole  is  inherited 
about  as  strongly  as  other  mental  and  physical  character- 
istics. 

But  insanity  cannot  be  regarded  as  a  simple  defect  wliicli 
can  accordingly  be  eliminated  from  a  population  altogether, 
as  could  albinism.  Insanity  is  a  general  name  for  a  great 
variety  of  conditions  of  mental  lack  of  balance  and  man>' 
different  factors  may  enter  into  it.  Not  every  family  stock 
in  which  it  occurs  is  to  be  regarded  as  unsound.  15ut  the 
intermarriage  of  families  in  which  insanity  occurs,  and,  still 
more,  inbreeding  within  a  family  containing  insanity  is  likely 
to  increase  the  percentage  of  insane  offspring  and  so  should 
be  avoided. 

Two  American  investigators  (Rosanoft'  and  Orr)  more 
friendly  than  the  biometric  school  to  iVIendclian  theory,  have 

1  Dr.  Fay's  U.  S.  data. 


282 


GENETICS  AND  EUGENICS 


attempted  to  eliminate  several  categories  of  insanity  and  to 
find  out  more  precisely  what  the  law  of  inheritance  of  the 
remaining  sort  is.  They  eliminate  cases  possibly  due  to 
injury  to  the  brain,  alcoholism,  syphilis,  tumors,  apoplexy 
and  the  like.  Their  material  consisted  of  cases  in  the  state 
hospital  for  the  insane  at  Kings  Park,  N.  Y.  Careful  in- 
quiry was  made  as  to  the  pedigree  of  all  patients  whose  in- 
sanity was  not  referable  to  other  than  genetic  causes. 
Seventy-two  families  were  thus  investigated,  representing 
two  hundred  and  six  different  matings,  with  a  total  of  one 
thousand  ninety-seven  offspring.  These  are  tabulated  to 
test  the  hypothesis  that  insanity  is  a  Mendelian  recessive 
unit-character,  as  follows: 


TABLE  35 
Data  on  Inheritance  of  Insanity  (Rosanoff  and  Orr) 


Mat- 
ings 

Children 

rarents 

Neuro- 
pathic 

Normal 

Expected 

Both  insane 

17 
93 
14 
62 

20 

54 
190 

•   • 

107 

10* 

239 

45 

215 

77 

All  insane. 

Only  one  insane,  DR  X  RR 

1:1 

Only  one  insane,  DD  X  RR 

All  sane. 

Both  normal  (but  tainted),  DR  X  DR 

Both  normal   (only  one  or  neither  tainted), 
DR  X  DD  (?) 

1:3 
All  sane. 

*  Eight  have  not  yet  passed  "  age  of  incidence." 

The  table  seems  in  a  general  way  to  substantiate  the 
hypothesis  advanced,  chat  insanity  is  a  recessive  character, 
especially  the  first  category  of  matings  where  only  insane 
progeny  are  expected.  But  when  we  look  into  the  method  of 
gathering  the  data  and  of  compiling  the  table  we  become 
somewhat  skeptical  of  this  conclusion.  The  data  have  the 
scientific  value  of  gossip,  consisting  of  answers  made  by  "  in- 
formants "  to  leading  questions  designed  to  bring  out  any 
weakness  in  the  pedigree.  Like  inquiries  made  concerning 
any  individual  in  the  community  would  show  him  an  un- 


INSANITY  283 

mistakable  victim  of  insanity.  The  authors  frankly  a^hnil 
that  "  of  the  four  hundred  and  thirty-seven  persons  classed 
by  them  as  neuropathic,  only  one  hundred  and  fifteen,  or  '2().S 
per  cent,  presented  at  any  time  in  their  lives  indications  for 
commitment  to  sanitariums  or  hospitals  for  the  insane." 
Three-fourths,  therefore,  of  their  persons  insane  for  pedi^Tce 
purposes  would  be  classed  as  fully  normal,  if  they  occurred  in 
families  free  from  insane  hospital  patients.  Such  classifica- 
tion has  little  scientific  value. 

In  dealing  with  the  pedigrees  the  authors  class  as  neuro- 
pathic persons  whose  only  offence,  aside  from  having  an  in- 
sane relative,  are  the  following:  "  Crank  ";  "  easily  excited, 
nervous  temperament'!;  "  very  nervous  " ;  "  erratic,  excit- 
able ";  "  nervous,  little  things  bothered  her,  worried  a  great 
deal  ";  but  in  one  case,  which  goes  beyond  all  others,  the 
individual  is  classed  as  insane  on  the  following  grounds: 
"  money  mad,  very  cruel,  very  miserly  though  wealthy,  left 
much  of  his  money  to  his  housekeeper."  To  the  layman  this 
does  not  read  like  the  characterization  of  an  insane  person; 
change  the  word  housekeeper  to  hospital  and  it  might  de- 
scribe a  philanthropist  and  captain  of  industry. 

It  seems  that,  in  the  light  of  this  investigation,  if  critically 
viewed,  and  in  the  light  of  Heron's  investigation,  very  doubt- 
ful whether  insanity  in  general  is  inherited  as  a  Mendel ian 
unit-character.  Very  likely  there  are  different  varieties  of 
insanity  independently  inherited.  That  insanity  is  inherited, 
however,  there  can  be  no  doubt.  Heron  quotes  Pearson's 
family  records  as  including  seventeen  cases  in  which  one  or 
both  parents  were  insane.  In  only  one  case  were  all  nienibeis 
of  the  family  who  attained  the  age  of  fifty  or  over  hoc  from 
insanity.  When  both  parents  were  insane,  Pearson's  records 
give  66  per  cent  of  insane  offspring;  when  only  one  parent 
was  insane,  forty  per  cent  of  the  oft'spring  were  insane,  where- 
as in  the  general  population  only  1  or  2  per  cent  are  insane. 
Hence  with  insanity  in  one  or  both  parents,  the  percent- 
age of  insane  progeny  increases;  on  this  all  investigators 
agree. 


284 


GENETICS  AND  EUGENICS 


The  practical  conclusion  is  obvious :  insane  persons  should 
not  be  permitted  to  marry;  indeed  legislation  forbids  this  in 
most  countries.  Further  it  would  be  well  to  avoid  marriage 
into  families  in  which  insanity  is  common.  It  need  not  be 
assumed,  however,  that  every  person  who  has  had  an  insane 
relative  is  an  unfit  mate.  For  such  a  conclusion,  if  enforced, 
would  soon  bring  human  breeding  to  a  standstill. 

Epilepsy.  As  regards  the  inheritance  of  epilepsy  and 
feeble-mindedness  the  evidence  is  much  clearer.    By  epilepsy 

TABLE   36 

Epilepsy  and  Feeble-Mindedness  in  Epileptic  Families 

{Davenport  and  Weeks) 


Children 

Parents 

Number  of 
Matings 

Epileptic 

Feeble- 
Minded 

Normal 

Both  enileDtic 

1 
5 
6 
3 

3 

8 
5 

1 

6 

16 

4 

One  epileptic,  one  feeble-minded  .... 

Both  feeble-minded 

One  epileptic,  one  insane 

9* 

*  One  "  nem'otic." 

we  understand  such  nervous  troubles  as  manifest  themselves 
in  the  simplest  cases  in  momentary  loss  of  consciousness,  and 
in  extreme  cases  in  marked  convulsions.  Much  so-called 
epilepsy  is  probably  due  to  infection  with  syphilis,  congenital 
or  otherwise,  in  which  case  its  inheritance  would  be  apparent 
only. 

But  if  we  leave  out  of  account  this  possible  complication, 
the  inheritance  seems  to  be  that  of  a  simple  recessive  Men- 
delian  character.  Davenport  and  Weeks  (Eugenics  Record 
Office,  Bull.  No.  4)  have  tabulated  records  concerning  in- 
mates of  the  New  Jersey  State  Village  for  Epileptics  at  Skill- 
man,  N.  J.,  which  show  one  case,  in  which,  both  parents 
being  epileptic,  their  three  children  were  epileptic  also.  In 
^Ye  matings  between  an  epileptic  and  a  feeble-minded  person 
fourteen  children  were  produced,   eight  epileptic  and  six 


I 


FEEBLE-MINDEDNESS  285 

feeble-minded.  In  six  cases  feeble-minded  persons  marritnl 
each  other  producing  sixteen  feeble-minded  and  five  epileptic 
offspring.  These  cases  indicate  that  the  epilepsy  and  feeble- 
mindedness here  dealt  with  were  merely  different  manifes- 
tations due  to  a  single  cause,  either  a  common  infection  or 
a  common  form  of  defect  inherited  without  specific  infection. 

That  insanity  is  probably  due  to  a  variety  of  causes  and 
not  the  same  ones  as  epilepsy  or  feeble-mindedness  is  shown 
by  matings  of  the  insane  with  epileptic  or  feeble-minded  })er- 
sons.  Davenport  and  Weeks  report  three  matings  of  an  in- 
sane person  with  an  epileptic  or  feeble-minded  person,  which 
produced  fifteen  adult  offspring.  Of  these  nine,  or  a  majority, 
are  described  as  normal,  one  as  epileptic,  and  four  as  fee))le- 
minded,  while  one  is  classed  as  "  neurotic."  This  result  indi- 
cates that  the  insane  parent  in  most  of  these  cases  did  not 
transmit  the  same  abnormality  or  pathological  condition  as 
the  epileptic  or  feeble-minded  parent.  Insanity  in  the  family 
is  racially  less  serious  than  epilepsy,  possibly  because  less 
often  due  to  congenital  infection. 

Feeble-mindedness.  The  most  complete  study  of  the  inheri- 
tance of  feeble-mindedness  that  has  ever  been  made  is  that 
published  by  Dr.  H.  H.  Goddard  of  the  Vineland  New  Jersey 
Training  School  for  Feeble-minded,  who  has  recently  pub- 
lished his  results  in  book  form  (Macmillan  &  Co.,  1914).  He 
has  studied  the  family  histories  of  three  hundred  and  twenty- 
seven  families  which  sent  pupils  to  the  Vineland  School. 
These  family  histories  are  pubHshed  in  detail,  though  not 
of  course  by  name,  and  include  in  many  cases  photographs 
of  the  pupil  or  of  his  written  work.  In  every  case  the  family 
pedigree  is  charted  to  show  the  occurrence  of  mental  or 
physical  peculiarities  in  ancestors  or  any  pertinent  facts  con- 
cerning their  lives.  The  information  was  obtained  from  the 
parents  of  pupils,  from  family  physicians,  friends  or  neigh- 
bors, partly  through  printed  questionnaires,  partly  throngh 
personal  interviews  by  trained  investigators.  This  method 
of  obtaining  information  is  of  course  capable  of  uncritical  use, 
as  already  pointed  out,  but  seems  to  have  been  employed 


286  GENETICS  AND  EUGENICS 

with  circumspection  and  in  some  cases  with  independent 
verification  by  Dr.  Goddard. 

The  importance  of  such  an  investigation  as  this  is  shown, 
according  to  Goddard,  by  many  facts. 

First.  Feeble-mindedness  is  much  commoner  than  most 
persons  suppose,  understanding  the  feeble-minded  to  include 
all  persons  congenitally  of  such  low  intelligence  that  they  are 
either  unable  to  care  for  themselves  or  are  incapable  of 
managing  their  own  affairs  with  ordinary  prudence.  God- 
dard believes  that  the  feeble-minded  are  individuals  of 
arrested  or  undeveloped  mentality  and  are  thus  quite  differ- 
ent from  the  insane,  who  show  pathological  mentality.  A 
feeble-minded  person  has  the  undeveloped  mind  of  a  child; 
an  insane  person  may  have  attained  mental  maturity  and 
then  lost  it  again,  his  mentality  having  degenerated.  Feeble- 
mindedness and  insanity  may  coexist  in  the  same  individual 
but  they  are  due  to  distinct  agencies.  Feeble-mindedness, 
according  to  Goddard,  characterizes  a  large  proportion  of 
such  persons  as  become  public  charges  as  paupers,  drunkards, 
or  criminals. 

The  method  now  generally  employed  of  grading  the  intelli- 
gence of  individuals  is  known  as  the  Binet  test,  from  the 
Frenchman  who  devised  it.  It  consists  of  giving  the  indi- 
vidual a  series  of  standardized  tasks  to  perform  of  increasing 
difficulty  as  regards  the  demands  on  intelligence.  The  results 
of  these  tests  are  graded  in  terms  of  the  average  performance 
of  normal  children  of  particular  ages.  Thus  a  feeble-minded 
person  may  show  the  mentality  of  a  normal  child  of  any  age 
from  one  year  to  twelve  years,  and  is  spoken  of  as  mentally 
of  age  one,  two,  three,  etc.  Tests  of  intelligence  made  by  the 
Binet  method  upon  juvenile  criminals  in  various  state  refor- 
matories show  that  a  large  proportion  of  the  inmates  are  of 
abnormally  low  intelligence,  i.  e.,  are  feeble-minded.  In 
New  Jersey  the  proportion  reported  feeble-minded  as  indi- 
cated by  Binet  tests  is  46  per  cent;  in  Ohio  70  per  cent;  in 
Virginia  79  per  cent;  and  in  Illinois  89  per  cent.  Probably 
50  per  cent  would  be  a  conservatively  low  general  estimate 


FEEBLE-MINDEDNESS  !287 

of  the  youthful  crimmals  who  are  feeble-iniiuk-d.  Goddard 
says,  "  It  is  easier  for  us  to  realize  this  if  we  remember  how 
many  of  the  crimes  that  are  commited  seem  fooHsh  and  sill  v. 
One  steals  something  that  he  cannot  use  and  cannot  dispose 
of  without  getting  caught.  A  boy  is  offended  because  llic 
teacher  will  not  let  him  choose  what  he  will  study,  and 
therefore  he  sets  fire  to  the  school  building.  Another  kills  a 
man  in  cold  blood  in  order  to  get  two  dollars.  Somebody-  v\sv 
allows  himself  to  be  persuaded  to  enter  a  house  and  pass  out 
stolen  goods  under  circumstances  where  even  slight  intelli- 
gence would  have  told  him  he  was  sure  to  be  caught.  Some- 
times the  crime  itself  is  not  so  stupid  but  the  perijotrator  acts 
stupidly  afterwards  and  is  caught,  where  an  intelligent  per- 
son would  have  escaped.  Many  of  the  *  unaccountable  ' 
crimes,  both  large  and  small,  are  accounted  for  once  it  is 
recognized  that  the  criminal  may  be  mentally  defective. 
Judge  and  jury  are  frequently  amazed  at  the  folli/  of  the 
defendant  —  the  lack  of  common  sense  that  he  displayed  in 
his  act.  It  has  not  occurred  to  us  that  the  folly,  the  crudity, 
the  dullness,  was  an  indication  of  an  intellectual  trait  tliat 
rendered  the  victim  to  a  large  extent  irresponsible." 

This  same  line  of  explanation  Goddard  applies  with  much 
plausibility  to  drunkenness  in  relation  to  feeble-mindedness. 
It  is  well  known  that  drunkenness  and  feeble-mindedness  are 
often  associated,  and  people  have  concluded  that  drunken- 
ness causes  feeble-mindedness.  Goddard  believes  the  reverse 
of  this  to  be  true  that  feeble-mindedness  occasions  drunken- 
ness, because  the  individual  has  not  enough  intelligence  and 
will  power  to  resist  temptation  when  it  arises. 

Another  social  evil,  prostitution,  Goddard  finds  to  l)e  due 
in  large  measure  to  feeble-mindedness.  Binet  tests  nuide  in 
an  Illinois  reformatory  of  girls  committed  for  innnorality 
showed  97  per  cent  of  them  to  be  feeble-minded.  A  ^Fassa- 
ehusetts  Commission  reports  that  Binet  tests  api)lie(l  to 
three  hundred  immoral  women  under  detention  in  that  state 
proved  51  per  cent  of  them  to  be  feeble-minded,  while  tlie 
rest  had  the  mentality  of  children  aged  nine  to  twelve  years. 


288  GENETICS  AND  EUGENICS 

If  Dr.  Goddard  is  right  in  the  opinion  that  feeble-minded- 
ness  is  responsible  for  much  crime  of  various  sorts,  for  much 
drunkenness  and  pauperism,  it  would  seem  that  the  easiest 
way  to  attempt  to  diminish  these  evils  would  be  by  attempt- 
ing to  diminish  feeble-mindedness.  Hence  the  importance 
of  his  undertaking  to  get  at  the  causes  of  feeble-mindedness. 

Dr.  Goddard  divides  his  three  hundred  and  twenty-seven 
cases,  as  regards  the  probable  causes  of  the  observed  feeble- 
mindedness, into  six  groups: 

1.  Hereditary 164 

2.  Probably  hereditary 34 

3.  Neuropathic  ancestry  (a  possible  cause) 37 

4.  Accident  (to  mother  or  child,  as  disease) 57 

5.  No  cause  assignable 8 

6.  Unclassified 27 

327 

From  this  table  it  will  be  seen  that  he  regards  the  feeble- 
mindedness as  clearly  hereditary  in  half  of  the  families 
studied,  while  it  is  "  probably  hereditary  "  in  10  per  cent 
more.  Heredity  then  is  the  largest  single  discoverable  cause 
for  feeble-mindedness.  Neuropathic  ancestry  and  accident 
are  also  recognized  as  probable  causes  in  a  small  percentage 
of  cases  each,  but  it  is  not  to  be  expected  that  feeble-minded- 
ness so  produced  would  prove  hereditary.  He  can  find  no 
evidence  that  hereditary  feeble-mindedness  is  caused  by  a 
variety  of  agencies  to  which  it  is  frequently  referred,  as  for 
example  to  alcoholism,  tuberculosis,  syphilis,  insane,  epilep- 
tic or  paralytic  ancestry,  etc. 

Most  feeble-mindedness,  then,  is  due  to  heredity,  but  how 
did  the  character  become  hereditary  ?  How  did  it  originate  ? 
Goddard  does  not  attempt  to  answer  this  question,  but  he 
does  make  clear  his  view  that  the  feeble  mind  is  an  unde- 
veloped childish  mind.  His  observations  show  that  the 
physical  vigor  of  the  feeble-minded  equals  that  of  normal 
individuals  and  that  the  feeble-minded  are  even  more  fecund 
than  normal  individuals  owing  to  their  lack  of  normal  pru- 
dence and  self-control.    It  might  be  supposed,  therefore, 


FEEBLE-MINDEDNESS  281) 

either  that  they  represent  a  primitive,  animal-like  eonditiuu 
of  the  human  race,  which  has  survived  down  to  the  present 
time,  or  that  they  represent  a  retrogressive  (loss)  variation. 
The  manner  of  inheritance  of  the  condition  is  of  iiitiTesl  in 
connection  with  this  question,  for  evolution  by  loss  usually 
results  in  the  production  of  recessive  variati(jns. 

Goddard's  evidence  indicates  that  feeble-mindednrss  i>  a 
recessive  unit-character.  In  his  family  records  one  hundred 
and  forty-four  matings  of  feeble-minded  inter  se  have  j)ro- 
duced  seven  hundred  and  forty-nine  children  of  whom  four 
hundred  and  eighty-two  are  of  ascertained  mentality.  Of 
these,  all  but  six  are  recorded  as  feeble-minded.  These  few- 
exceptions  to  theoretical  expectation  might  be  explained  as 
being  of  ancestry  other  than  that  assigned.  A  case  reported 
from  an  Ohio  institution  illustrates  the  point  well.  "In  a 
white  family,  the  father  and  mother  are  both  feeble-minded. 
They  have  twelve  children,  all  feeble-minded  but  two.  These 
two  are  normal  (as  regards  intelligence)  but  they  are  colored." 

TABLE  37 
Data  on  the  Inheritance  of  Feeble-Mindedness 

Children 
Mating  F-M  N 

F  X  F  476  6 

F  X  N  193  144  (N   heterozygous  ?).      Some    families   tabulated 

here  belong  above,  probably. 

F  X  N  . .  68  (N  homozygous  ?). 

NXN  39  83  (Both  heterozygous  ?).    Some  belong  above,  prol>- 

ably. 

NXN  . .  116  (One  or  both  homozygous  ?). 

The  data  of  Goddard  indicate  clearly  that  feeble-mindedness 
is  inherited  as  a  recessive  Mendelian  character,  but  one  which 
like  albinism  may  occur  in  many  different  grades,  the  higluT 
grades  probably  tending  to  dominate.  The  feeblr-niindt-d 
are  frequently  deficient  in  physical  strength  and  vigor.  II  o w- 
ever,  many  of  them  seem  to  possess  unusually  good  physic lue. 
Goddard  compares  them  to  savages  with  strong  bodies  but 
childish  minds.  The  high-grade  feeble-minded,  kncnvn  as 
"  morons,"  with  mentality  of  eleven  or  twelve  years,  are 


290  GENETICS  AND  EUGENICS 

capable  of  being  useful  members  of  society  in  manual  or 
mechanical  occupations  not  demanding  too  much  planning 
or  initiative.  But  it  is  evident  that  as  they  are  easily  in- 
fluenced and  imposed  upon  and  more  than  ordinarily  fecund, 
since  they  do  not  exercise  the  prudence  and  self-restraint  of 
normal  individuals,  their  numbers  are  likely  to  increase  un- 
duly, unless  some  restraint  is  put  upon  them.  A  self-govern- 
ing democracy  with  universal  suffrage  is  seriously  threatened 
by  a  large  increase  in  the  unintelligent  portion  of  its  popula- 
tion, and  is  justified  in  adopting  strong  measures  to  counter- 
act it.  This  is  often  urged  as  an  argument  for  restricted  immi- 
gration without  due  regard  for  the  distinction  between  low 
intelligence  and  illiteracy.  Many  of  our  immigrants  w^ho  are 
illiterate,  because  they  have  never  had  an  opportunity  to 
attend  school,  are  people  of  unusual  intelligence  and  energy. 
Their  illiteracy  is  usually  speedily  removed  when  they  get 
within  reach  of  American  schools  and  the  next  generation  is 
represented  among  the  most  earnest  students  in  our  univer- 
sities and  later  among  the  successful  men  in  the  professions. 
But  the  person  of  low  intelligence,  whether  literate  or  illiter- 
ate is  more  dangerous  to  society  than  the  intelligent  illiterate, 
because  he  and  his  descendants  for  all  time  will  require 
parental  protection  and  care  from  the  state  to  prevent  them 
from  becoming  criminals,  paupers,  idlers,  and  purchasable 
voters. 

To  prevent  the  natural  increase  of  the  feeble-minded,  God- 
dard  recommends  their  segregation,  so  far  as  possible,  in 
schools  and  institutions  under  state  control.  This  is  already 
being  done  to  some  extent  in  many  of  the  states,  but  alto- 
gether too  few  individuals  have  yet  been  segregated  to  insure 
a  decrease  in  the  proportion  of  feeble-minded  in  the  popula- 
tion. Many  have  hitherto  been  unrecognized  as  feeble- 
minded, who  are  classed  as  backward  pupils  in  school,  and 
later  as  truants,  drug  fiends,  drunkards,  criminals,  tramps  or 
prostitutes.  A  proper  recognition  of  the  source  from  which 
these  classes  are  recruited  and  of  what  really  ails  them 
should  lead  to  more  intelligent  efforts  to  reduce  their  number. 


FEEBLE-MINDEDNESS 


291 


When  segregation  is  impracticable,  tlie  feeble-iniiKlcd  should 
be  looked  after  in  their  homes,  as  children  arc  looked  alter. 
They  should  not  be  allowed  to  marry  unless  first  sterilized. 
In  the  case  of  males  this  is  now  possible  by  a  very  siinj)le 
surgical  operation,  vasectomy,  unattended  by  risk  or  serious 
consequences  to  health.  In  the  case  of  females  se<,'regati()u 
during  the  reproductive  period  is  probably  more  to  be  recom- 
mended than  sterilization. 


CHAPTER  XXXIII 

THE  POSSIBILITY  AND  PROSPECTS  OF  BREEDING  A 

BETTER  HUMAN  RACE 

The  suggestion  that  the  human  race  might  be  improved  by 
the  methods  of  the  stock  breeder  is  a  very  old  one.  Plato 
advanced  it  in  his  Republic  as  the  only  practicable  basis  for 
the  production  of  a  permanent  and  superior  governing  class 
within  the  ideal  state.  The  family  had  no  place  in  his 
scheme. 

It  was  his  proposition  that  the  best  of  both  sexes  should 
be  mated  with  each  other  and  should  be  given  every  encour- 
agement to  the  production  of  offspring,  the  young  being 
taken  at  birth  into  a  state  nursery  and  their  identity  lost  so 
far  as  the  parents  were  concerned.  Inferior  persons,  on  the 
other  hand,  were  to  be  kept  from  reproducing,  as  far  as  pos- 
sible, and  their  progeny  destroyed.  Realizing  that  such 
favoritism  would  cause  no  end  of  trouble,  if  known,  Plato 
said  that  what  was  done  should  be  kept  a  secret  from  all  but 
the  magistrates  themselves,  and  "  an  ingenious  system  of  lots 
must  be  contrived  in  order  that  inferior  persons  may  impute 
the  manner  in  which  couples  are  united  to  chance  and  not  to 
the  magistrates." 

The  eugenics  system  of  Plato  has  probably  never  had  a 
full  and  fair  trial,  but  if  we  may  believe  the  account  of 
Plutarch,  in  his  life  of  Lycurgus,  something  very  like  it 
actually  existed  in  Plato's  time  in  Sparta,  and  it  was  prob- 
ably the  Spartan  system  that  Plato  had  in  mind.  Sparta  was 
practically  an  armed  camp,  in  which  a  military  class  ruled 
with  great  severity  the  subject  native  races,  holding  them  in 
subjection  by  force  of  arms  and  compelling  them  to  work  the 
land  for  the  benefit  of  their  conquerors.  The  Spartans  sub- 
jected themselves,  both  men  and  women,  to  the  severest 
discipline.     Gymnastics  and  war  were  their  exclusive  occu- 

292 


SPARTAN  EUGENICS  09:] 

pations.  Family  life  scarcely  existed  among  the  Spartans. 
The  men  lived  together  in  a  sort  of  camp  or  club,  very  fru- 
gally, and  ready  for  instant  warfare.  Marriage  was  recog- 
nized as  an  institution  for  the  production  of  sohh'crs  merely. 
The  child  belonged  to  the  state,  rather  than  to  its  pannts* 
The  magistrates  decided  whether  it  should  be  reared  or  not. 
Plutarch  says  concerning  Lycurgus,  founder  of  the  Si)artan 
constitution:— "Lycurgus  was  of  a  persuasion  that  children 
were  not  so  much  the  property  of  their  parents  as  of  the 
whole  commonwealth,  and  therefore,  would  not  have  his 
citizens  begot  by  the  first-comers,  but  by  the  best  men  that 
could  be  found;  the  laws  of  other  nations  seemed  to  him  very 
absurd  and  inconsistent,  where  people  would  be  so  solicitous 
for  their  dogs  and  horses  as  to  exert  interest  and  pay  money 
to  procure  fine  breeding,  and  yet  kept  their  wives  shut  up, 
to  be  made  mothers  only  by  themselves,  who  might  be  fool- 
ish, infirm,  or  diseased;  as  if  it  were  not  apparent  that  chil- 
dren of  a  bad  breed  would  prove  their  bad  qualities  first 
upon  those  who  kept  and  were  rearing  them,  and  well-born 
children,  in  like  manner,  their  good  qualities." 

The  Spartan  system  of  eugenics  seems  to  have  attained 
its  object,  the  production  of  superior  children,  but  we  must 
remember  that  with  it  was  combined  a  system  of  life-long 
physical  education  and  military  discipline  which  has  rarely 
if  ever  been  equalled,  so  that  it  is  impossible  to  say  how 
much  of  the  result  obtained  was  due  to  breeding  and  how 
much  to  training  of  the  youth. 

Further  the  Spartan  system  succeeded  only  so  long  as 
Sparta  was  a  small,  isolated  community,  without  wealth, 
luxury,  or  leisure,  and  using  iron  for  money.  Foreign  con- 
quest was  the  undoing  of  Sparta.  She  could  con(|uer  in  a 
fight  but  she  could  not  govern  except  as  she  governed  lier 
Helots  —  by  enslaving  them.  Upon  contact  with  the  rest  of 
the  world,  life  was  found  to  have  other  attractions  than 
fighting,  and  the  old  discipline  was  relaxed. 

Moreover,  what  the  Spartan  system  produced  wa.s  a  single 
type  of  man,  the  soldier.    The  memory  of  Athens  is  sacred  for 


294  GENETICS  AND  EUGENICS 

other  types  of  manhood  and  achievement,  art,  Hterature, 
philosophy  and  science,  the  greatest  intellectual  achieve- 
ments of  mankind  up  to  that  time,  but  in  these  Sparta  had 
no  share.  Her  eugenics  was  of  the  same  type  as  that  of  the 
animal  breeder.  It  aimed  to  produce  a  single  specialized  type 
of  superior  excellence.  In  this  it  succeeded,  but  at  the  sacri- 
fice of  all  else.  In  this,  again,  it  resembles  animal  husbandry, 
which  produces  a  type  of  animal  more  useful  to  man,  but 
wholly  dependent  upon  him,  and  unable  to  maintain  itself 
if  thrust  back  into  the  struggle  for  existence  with  other 
animals. 

The  civilization  for  whose  continuance  Plato  planned  came 
to  an  end.  We  do  not  know  why.  Historians  differ  widely 
in  their  views  as  to  why  Greece  and  Rome  fell.  But  one  sug- 
gestion is  that  in  their  later  days  the  inferior  classes  increased 
more  rapidly  than  the  superior  ones  and  the  general  average 
was  thereby  lowered.  Now  it  is  conceivable  that  this  may 
have  happened  in  one  of  two  ways.  If  each  class  reproduced 
its  kind,  then  the  lower  classes  must  have  reproduced  faster 
than  the  upper  ones.  This  is  what  is  assumed  to  have 
occurred  by  those  who  consider  modern  nations  to  be 
threatened  in  a  similar  way. 

On  the  other  hand  it  is  possible  that  there  was  no  real 
germinal  difference  between  the  so-called  upper  and  the 
lower  classes.  The  classification  of  ancient  society  may  have 
rested  on  economic  rather  than  biological  grounds  and  the 
downfall  have  been  due  to  economic  causes  rather  than  to 
racial  changes.  If  this  is  true  then  the  more  rapid  reproduc- 
tion of  those  low  in  the  social  scale  was  not  in  itself  harmful 
to  the  race,  that  is  would  not  have  caused  a  lowering  of  its 
biological  level,  and  economic  causes  must  be  sought  to 
explain  the  decay  of  ancient  civilization.  The  question  is 
one  for  historians  to  deal  with,  but  its  answer  must  be  borne 
in  mind  when  the  fate  of  ancient  civilizations  is  cited  as  a 
warning  to  us. 

A  belief  that  biological  decline  is  occurring  or  is  likely  to 
occur  among  modern  nations  has  given  rise  to  the  modern 


THE  MODERN  EUGENICS  MOVEMENT         29.5 

eugenics  movement.  This  movement  was  started  by  Francis 
Galton,  who,  adopting  Darwin's  theory  of  evolution,  souglit 
to  apply  it  to  human  society.  His  studies  of  family  histories 
had  convinced  him  that  both  physical  and  mental  traits  are 
largely  matters  of  inheritance.  He  reasoned  that  the  exist- 
ing biological  status  of  society  could  be  maintained  only  if  all 
classes  of  society  reproduced  at  the  same  rate;  that  improve- 
ment would  result  if  the  biologically  best  individuals  repro- 
duced faster  than  others,  but  that  deterioration  would  result 
if  the  biologically  inferior  individuals  reproduced  faster  than 
others.  He  sought  to  devise  measures  which  would  encourage 
early  marriage  and  the  rearing  of  large  families  by  the  best 
and  most  competent  members  of  every  profession  and  trade. 
His  suggestions  met  chiefly  with  ridicule  at  the  time,  but 
are  coming  now  to  be  taken  more  seriously. 

No  one  can  deny  that  our  country's  population  is  increas- 
ing fast  enough,  the  only  danger  is  that  the  biologically  poor- 
est elements  in  the  population  may  increase  faster  than  any 
other.  The  declining  birth  rate  is  not  in  itself  serious,  but 
the  differential  character  of  its  decline  is  serious.  The  most 
intellectual  and  cultured  elements  in  the  population  breed 
slowest.  Professor  Cattell  says  that  a  Harvard  graduate  has 
on  the  average  three-fourths  of  a  son  and  a  Vassar  graduate 
one-half  of  a  daughter.  If  this  continues  college  graduates 
may  look  forward  to  the  early  extinction  of  their  line  as  an 
element  in  the  American  population. 

As  elements  in  the  differentially  declining  birth-rate  we 
may  recognize  (1)  late  marriages,  shortening  the  reproductive 
period  and  (2)  voluntary  limitation  of  the  number  of  children. 
Voluntary  limitation  occurs  for  a  variety  of  reasons  such  as 
expense,  health,  etc.,  but  chiefly  because  of  selfishness  and 
luxury,  causes  which  were  operative  in  the  decline  of  Greece 
and  Rome  as  they  are  among  modern  nations. 

The  more  complex  human  life  becomes,  the  less  attention 
is  given  to  its  perpetuation.  In  a  small  connnunity  family 
life  is  dominant  and  the  rearing  and  education  of  children  are 
its  most  important  occupations.    But  as  comumnity  life  be- 


296 


GENETICS  AND  EUGENICS 


comes  more  complex  family  life  sinks  into  a  subordinate 
position.  The  more  intellectual  and  cultured  the  individual 
is,  the  more  does  he  find  outside  the  home  to  interest  and 
attract  him.  The  consequence  is  that  home  life  suffers.  It 
is  slighted  or  shunned  altogether  by  those  who  are  best  quali- 
fied to  be  parents,  and  the  rearing  of  children  is  left  to  those 
considered  too  dull  for  other  activities.  In  consequence  the 
majority  of  the  children  produced  in  a  cultured  and  progres- 
sive city  population  are  produced  by  its  least  cultured  and 
progressive  members.  This  is  the  condition  which  today 
confronts  the  leading  nations  of  the  world  and  has  given  rise 
to  the  eugenics  movement.  If  this  condition  is  interpreted 
from  the  standpoint  of  the  animal  breeder,  it  means  that  the 
average  capacity  of  the  population  for  intellectual  pursuits, 
for  culture  and  for  progress  is  bound  to  decline.  For  this 
amounts  to  selecting  for  breeding,  not  the  best,  but  the  culls 
of  the  flock,  and  every  breeder  knows  that  this  means 
deterioration. 

If  a  great  city  can  in  each  generation  import  a  fresh  stock 
of  youths  from  the  country  or  from  foreign  countries,  all 
may  go  well,  but  it  is  questionable  whether  this  can  continue 
indefinitely.  Already  many  of  our  rural  New  England  com- 
munities are  said  to  be  running  out  of  good  human  stock. 
For  generations  they  have  been  sending  their  best  to  the 
cities  and  to  the  developing  West.  Many  of  those  left  be- 
hind are  lacking  in  energy  or  ambition,  perhaps  also  in 
intelligence,  and  a  European  peasant  population  is  rapidly 
replacing  them.  Will  this  new  population  be  a  fit  substitute 
for  the  old  Anglo-Saxon  stock  ?  Time  alone  will  tell.  If  it 
is  a  sound  stock  which  has  hitherto  lacked  opportunity  to 
rise  in  the  social  scale,  we  may  now  expect  it  to  do  so,  oppor- 
tunity being  offered.  But  if  it  is  inherently  a  feeble  stock,  it 
will  not  replace  the  old  New  England  stock  in  supplying  our 
cities  with  the  bright  youths  whom  they  require  but  are  un- 
able to  produce  in  sufficient  numbers.  A  time  of  storm  and 
stress  like  that  which  now  distracts  the  world  may  at  some 
future  day  decide  our  fitness  to  survive  as  a  race. 


EUGENICS  IN  ENGLAND  297 

In  England  a  genuine  alarm  is  felt  as  regards  the  character 
of  its  future  citizens,  for  there  as  here  the  cities  draw  from 
the  country.  But  the  country  population  there  is  not  only 
not  regenerated  by  immigration  but  is  further  depleted  of  its 
best  elements  by  foreign  emigration.  The  consequence  i^ 
that  a  eugenics  movement  has  there  been  started,  which 
seeks  to  remove  the  indifference  on  the  part  of  the  best  ele- 
ments in  the  population  to  marriage  and  the  rearing  of  chil- 
dren. Just  how  this  can  be  done,  or  whether  it  can  be  done 
at  all  is  uncertain.  But  the  British  eugenists  are  very  much 
in  earnest  and  they  base  their  appeal  on  both  patriotic  and 
religious  grounds.  Professor  and  Mrs.  Whetham  (who  have 
written  several  books  devoted  to  this  subject)  discuss  pri- 
marily conditions  in  Great  Britain.  Their  point  of  view 
is  to  some  extent  an  aristocratic  one.  They  recognize  in  the 
hereditary  aristocracy  of  England  a  genuinely  and  gemiinally 
superior  element  of  the  population.  The  younger  sons  of 
the  titled  families  who  inherit  (it  is  supposed)  the  superior 
germ-plasm  but  not  the  aristocratic  titles,  have  frequently 
married  into  successful  families  of  the  middle  class,  and 
are  believed  thus  to  have  improved  the  standard  of  the 
entire  nation.  This  theory  sounds  plausible,  but  an  out- 
sider free  from  class  prejudice  might  reasonably  question 
its  validity. 

If  the  English  aristocracy  is  really  a  biologically  superior 
race,  how  are  we  to  account  for  the  historical  steady  rise  in 
power  and  influence  of  the  Commons  ^  Opportunity  has 
always  favored  the  aristocratic  families;  in  spite  of  this  we 
find  the  great  men  of  the  British  nation  usually  coming  from 
the  middle  class,  and  not  from  the  younger  sons  of  aristo- 
cratic families  either.  America's  experience  does  not  indicate 
that  the  English  aristocracy  is  either  better  or  worse  than  the 
English  yeomanry  as  a  biological  human  stock.  What  little 
of  aristocratic  blood  the  colonies  received  went  chiefly  to 
Virginia  and  previous  to  the  Civil  War  an  aristocracy  of  first 
families  comparable  with  that  of  England  ruled  \^irginia  and 
furnished  the  nation  with  presidents  and  statesmen.    Since 


298  GENETICS  AND  EUGENICS 

the  war  the  presidents  have  come  from  other  sections,  and 
seem  not  to  have  been  inferior  in  ability  to  their  predecessors. 
In  some  quarters  it  is  the  fashion  to  point  to  New  England 
as  the  source  of  the  really  superior  American  stock,  viz.,  its 
intellectuals,  but  there  is  no  better  ground  for  thinking  the 
Puritan  stock  superior  than  for  thinking  the  Cavalier  stock 
superior.  Circumstance  has  had  much  to  do  with  the  ad- 
vancement of  each  in  influence.  In  this  connection  it  is 
interesting  to  note  the  conclusions  reached  by  Professor 
Cattell  {Popular  Science  Monthly,  May,  1915)  from  a  study  of 
the  families  of  America's  one  thousand  leading  scientists.  He 
says : 

"If  men  of  performance  could  only  come  from  superior  fam- 
ily lines,  this  would  be  a  conclusive  argument  for  a  privileged 
class  and  for  a  hereditary  aristocracy.  If  the  congenital 
equipment  of  an  individual  should  prescribe  completely  what 
he  will  accomplish  in  life,  equality  of  opportunity,  education 
and  social  reform  would  be  of  no  significance.  Such  an  ex- 
treme position,  though  it  is  approached  by  men  with  so  much 
authority  as  Sir  Francis  Galton,  Professor  Karl  Pearson,  Dr. 
F.  A.  Woods,  Dr.  C.  B.  Davenport  and  Professor  E.  L. 
Thorndike,  is  untenable.  Equally  extreme  in  the  opposite 
direction  is  M.  Odin's  aphorism  "  Genius  is  in  things  not  in 
men,"  or  the  not  uncommon  opinion  that  almost  anything 
can  be  done  with  a  child  by  training  and  education. 

My  data  show  that  a  boy  born  in  Massachusetts  or  Con- 
necticut has  been  fifty  times  as  likely  to  become  a  scientific 
man  as  a  boy  born  along  the  southeastern  seaboard  from 
Georgia  to  Louisiana.  They  further  show  that  a  boy  is  fifty 
times  as  likely  to  do  scientific  work  as  a  girl.  No  negro  in 
this  country  has  hitherto  accomplished  scientific  work  of 
consequence.  A  boy  from  the  professional  classes  in  New 
England  has  a  million  chances  to  become  a  scientific  leader 
as  compared  with  one  chance  for  a  negro  girl  from  the 
cotton  fields. 

"  These  great  differences  may  properly  be  attributed  in  part 
tO  natural  capacity  and  in  part  to  opportunity.    If  the  174 


CATTELL  ON  EUGENICS  ^99 

» 

babies  born  in  Massachusetts  and  Connecticut  who  became 
leading  scientific  men  had  been  exchanged  with  babies  born 
in  the  south,  it  seems  probable  that  few  or  none  of  them 
would  have  become  scientific  men.  It  may  also  be  the  case 
that  few  or  none  of  the  babies  from  the  south  transplanted  to 
New  England  would  have  become  scientific  men,  ])ut  it  is 
probably  true  that  a  nearly  equal  number  of  scientific  men 
would  have  been  reared  in  New  England.  It  is  certain  that 
there  would  not  have  been  174  leading  scientific  men  from 
the  extreme  southern  states  and  practically  none  from  Massa- 
chusetts and  Connecticut.  If  the  stock  of  the  southern 
states  remains  undiluted,  it  may,  as  social  conditions  change, 
produce  even  more  scientific  men  per  thousand  of  its  popula- 
tion than  New  England  has  hitherto  produced.  In  the  first 
list  [made  in  1906]  of  the  thousand  leading  scientific  men, 
Massachusetts  produced  109  and  Connecticut  87  per  million 
of  their  population.  Of  the  younger  men  added  to  the  list 
in  the  second  arrangement  [made  in  1910]  under  comparable 
conditions,  Massachusetts  produced  85  and  Connecticut  57. 
The  other  North  Atlantic  states  failed  in  like  measure,  while 
the  central  states  show  a  gain  —  Michigan  from  36  to  74, 
Minnesota  from  23  to  59,  etc.  These  changes  must  be  attri- 
buted to  an  altered  environment,  not  to  an  altered  racial 
stock.  Japan  had  no  scientific  men  a  generation  ago  and 
China  has  none  now,  but  it  may  be  that  in  a  few  years  their 
contributions  to  science  will  rival  ours. 

"A  Darwin  born  in  China  in  1809  could  not  have  become  a 
Darwin,  nor  could  a  Lincoln  born  here  on  the  same  day  have 
become  a  Lincoln  had  there  been  no  civil  war.  If  the  two 
infants  had  been  exchanged  there  would  have  been  no  Dar- 
win in  America  and  no  Lincoln  in  England.  Darwin  was  a 
member  of  a  distinguished  family  line  possessing  high  natural 
ability  and  the  advantages  of  opportunity  and  wealth.  Lin- 
coln had  no  parental  inheritance  of  ability  or  wealth,  but  he 
too  had  innate  capacity  and  the  opportunity  of  circum- 
stance. If  no  infants  had  been  born  with  the  peculiar  natural 
constitutions  of  Darwin  and  Lincoln,  men  like  them  could 


300  GENETICS  AND  EUGENICS 

not  have  been  made  by  any  social  institutions,  but  none  the 
less  the  work  they  did  might  have  been  accomplished  by 
others  and  perhaps  their  fame  would  have  been  allotted  to 
others.  There  may  have  been  in  England  other  family  lines 
equal  in  natural  ability  to  the  Darwins  and  in  this  country 
other  individuals  as  well  constituted  as  Lincoln,  but  undis- 
tinguished from  lack  of  opportunity.  It  is  still  more  probable 
that  such  conditions  obtain  in  Russia  and  in  China,  in  whose 
graveyards  there  may  lie  innumerable  *'  mute  inglorious  " 
Miltons,  Lincolns  and  Darwins. 

"The  most  exceptional  ability  may  be  suppressed  by  cir- 
cumstances; but  it  can  sometimes  deal  with  them  on  equal 
or  perhaps  superior  terms.  Thus  the  writer  has  pointed  out 
how  widely  distributed  in  race,  age  and  performance  are  the 
most  distinguished  men  who  have  lived.  When  we  turn 
from  the  most  eminent  men  to  those  next  in  rank,  we  may 
doubt  whether  their  natural  ability  has  not  been  equaled  by 
thousands  who  have  not  attained  distinction.  Among  the 
two  hundred  most  eminent  men  who  have  lived  in  the  his- 
tory of  the  world  are:  Napoleon  III,  Nero,  Fox,  Julian, 
Fenelon,  Clive,  Alberoni,  Bentley  and  Gerson.  It  is  quite 
conceivable  that  there  are  at  present  living  in  the  United 
States  hundreds  or  thousands  of  men  having  as  great  natural 
ability  as  these.  There  may  be  a  hundred  thousand  men  and 
women  having  the  natural  and  specific  ability  of  the  thou- 
sand in  this  country  who  have  accomplished  the  best 
scientific  work. 

"President  A.  Lawrence  Lowell  has  remarked  that  we  have 
a  better  chance  of  rearing  eaglets  from  eagles'  eggs  placed 
under  a  hen  than  from  hen's  eggs  placed  in  an  eagle's  nest. 
But  it  is  equally  true  that  we  have  a  better  chance  of  raising 
tame  eaglets  in  a  chicken  coop  than  in  an  eyrie.  The  differ- 
ence between  a  man  uninterested  in  science  and  a  scientific 
man  is  not  that  between  a  chicken  and  an  eagle,  but  that 
between  an  untrained  chicken  and  a  trick  cock.  Some 
cockerels  can  be  trained  better  than  others,  but  there  are  in- 
numerable cockerels  that  might  be  trained  and  are  not. 


CATTELL  ON  EUGENICS  301 

**The  son  of  a  scientific  man  may  on  the  average  have  the 
inherited  ability  which  would  make  him  under  equally  favor- 
able circumstances  twice,  or  ten  times,  or  a  hundred  times, 
as  likely  to  do  good  scientific  work  as  a  boy  taken  at  random 
from  the  community.  The  degree  of  advantage  should  be 
determined.  It  surely  exists,  and  the  children  of  scientific 
men  should  be  numerous  and  well  cared  for.  But  we  can  do 
even  more  to  increase  the  number  of  productive  scientific  men 
by  proper  selection  from  the  whole  community  and  by  giving 
opportunity  to  those  who  are  fit.  Galton  finds  in  the  judges 
of  England  a  notable  proof  of  hereditary  genius.  It  would  be 
found  to  be  much  less  in  the  judges  of  the  United  States.  It 
could  probably  be  shown  by  the  same  methods  to  be  even 
stronger  in  the  families  conducting  the  leading  publishing 
and  banking  houses  of  England  and  Germany.  As  I  write, 
the  death  is  announced  of  Sir  William  White,  the  distin- 
guished naval  engineer,  chief  constructor  of  the  British  navy, 
president  of  the  British  Association.  If  his  father  had  been 
chief  constructor  of  the  navy,  he  would  have  been  included 
among  Galton's  noteworthy  families  of  fellows  of  the  Royal 
Society.  The  fact  that  his  father-in-law  was  chief  construc- 
tor of  the  British  navy  throws,  if  only  by  way  of  illustration, 
a  light  on  the  situation  in  two  directions. 

On  the  one  hand,  the  specific  character  of  performance  and 
degree  of  success  are  determined  by  family  position  and 
privilege  as  well  as  by  physical  heredity;  on  the  other  hand, 
marriage,  chiefly  determined  by  environment,  is  an  import- 
ant factor  in  maintaining  family  lines.  The  often-quoted 
cases  of  the  Jukes  and  Edwards  families  are  more  largely  due 
to  environment  and  intermarriage  within  that  environment 
than  to  the  persistence  of  the  traits  of  one  individual  through 
several  generations.  The  recently  published  *'  Kallikak 
Family  "  by  Dr.  H.  H.  Goddard  demonstrates  once  again  the 
heredity  of  feeble-mindedness.  It  would,  however,  have  been 
a  stronger  argument  for  the  omnipotence  of  heredity  if  the 
original  ancestor  had  left  by  a  healthy  mother  illegitimate 
children  who  established  prosperous  lines  of  descent,  and  a 


302  GENETICS  AND  EUGENICS 

child  by  a  feeble-minded  wife  who  left  degenerate  lines  of 
descent.  Two  experiments  have  been  made  on  a  large  scale 
which  seem  fairly  definite  even  though  quantitative  results 
cannot  at  present  be  reached.  The  mulattoes  may  be  as- 
sumed to  have  a  heredity  midway  between  negroes  and 
whites,  but  their  social  environment  is  that  of  the  negroes, 
and  their  performance  corresponds  with  their  social  environ- 
ment rather  than  with  their  heredity.  Illegitimate  children 
have  perhaps  a  heredity  as  good  as  the  average,  but  their 
performance  falls  far  below  the  average.  If  performance 
were  determined  by  heredity  alone  there  might  be  expected 
to  be  among  our  thousand  leading  scientific  men  some  forty 
mulattoes  and  some  forty  of  illegitimate  birth,  whereas  there 
is  probably  not  one  of  either  class. 

"At  nearly  the  same  time  Agassiz  came  from  abroad  to 
Harvard  and  Brlinnow  to  Michigan.  We  all  know  the  list 
of  distinguished  naturalists  trained  under  Agassiz  —  Brooks, 
Hyatt,  Jordan,  Lyman,  Minot,  Morse,  Packard,  Putnam, 
Scudder,  Shaler,  Verrill,  Whitman,  Wilder,  and  many  more, 
directly  and  indirectly.  From  Michigan  have  come,  as  is  not 
so  well  known,  one-fourth  of  our  most  distinguished  astron- 
omers, including  Abbe,  Campbell,  Comstock,  Curtis,  Doo- 
little.  Hall,  Hussey,  Klotz,  Leuschner,  Payne,  Schaeberle, 
Watson  and  Woodward.  Certainly  the  coming  of  Agassiz 
and  Brunnow  was  the  real  cause  of  greatly  increased  scientific 
productivity  in  America.  Some,  but  not  all,  of  those  who 
worked  under  Agassiz  would  have  become  naturalists  apart 
from  his  influence.  The  astronomers  from  Michigan  must  in 
the  main  be  attributed  to  their  environment.  The  men  had 
the  necessary  ability,  but  if  Brunnow  had  not  gone  to  Michi- 
gan, they  would  not  have  become  astronomers;  if  they  had 
gone  to  the  University  of  Pennsylvania,  they  would  have 
been  more  likely  to  have  become  physicians  than  astrono- 
mers; if  they  had  not  gone  to  a  university  they  would  not 
have  become  scientific  men. 

*'It  is  certainly  satisfactory  if  we  can  attribute  the  inferi- 
ority of  scientific  performance  in  America  as  compared  with 


CATTELL  ON  EUGENICS  303 

Germany,  France  and  Great  Britain  to  lack  of  opportunity 
rather  than  to  lesser  racial  ability.  In  Germany  scientific 
research  has  been  made  by  the  university  rather  than  the 
reverse.  In  Great  Britain  also  the  universities  have  been 
potent,  and,  in  addition,  its  leisure  class  has  conlrihulcd 
greatly.  Here  prior  to  1876  we  had  no  university  in  wliicli 
research  work  was  adequately  encouraged,  and  we  have  had 
no  amateurs  comparable  to  those  of  Great  Britain.  Professor 
Pickering  found  that  of  the  87  scientific  men  who  were  mem- 
bers of  at  least  two  foreign  academies,  6  were  Americans  as 
compared  with  17  from  Prussia,  13  from  England  and  12 
from  France.  In  so  far  as  our  scientific  production  is  so 
measured,  the  reference  is  to  a  generation  ago,  when  our 
universities  were  only  beginning  to  develop  and  research 
work  was  only  beginning  to  be  appreciated.  But  it  is  a  strik- 
ing fact  that  of  the  six  distinguished  Americans,  three  are 
astronomers;  and  astronomy  is  the  only  science  in  which 
thirty  years  ago  the  facilities  for  research  work  in  this  coun- 
try were  equal  to  those  of  the  leading  European  nations.  Of 
the  remaining  three,  two  have  not  been  engaged  in  teaching, 
and  the  third  has  been  practically  freed  from  teaching  for  his 
research  work.  We  may  hope  that  when  conditions  become 
as  favorable  for  other  sciences  as  they  have  been  for  astron- 
omy, the  United  States  will  assume  leadership  in  scientific 
productivity. 

"In  order  to  answer  questions  such  as  the  extent  to  which 
the  scientific  work  accomplished  in  America  is  due  to  native 
endowment,  whether  such  endowment  is  general  or  specific, 
how  far  it  occurs  in  family  lines,  what  part  of  those  endowed 
are  able  to  prove  their  ability,  the  influence  of  education  and 
example,  the  effects  of  opportunity,  encouragement  and  re- 
wards, it  is  necessary  to  make  a  study  of  individual  cases. 
A  large  mass  of  material  is  at  hand  concerning  the  relatives  of 
scientific  men  who  have  shown  scientific  productivity  or  have 
attained  distinction,  but  these  data  are  not  in  order  for  pub- 
lication and  should  be  supplemented  by  answers  to  many 
inquiries.     In  the  meanwhile  the  writer  may  say  that  it  is 


304  GENETICS  AND  EUGENICS 

his  opinion  that  while  we  should  welcome  and  support  a 
eugenic  movement  tending  to  limit  the  birth  of  feeble-minded 
and  defective  children  and  encouraging  the  birth  of  those 
that  are  well  endowed,  it  appears  that  under  the  existing 
conditions  of  knowledge,  law  and  sentiment,  we  can  probably 
accomplish  more  for  science,  civilization  and  racial  advance 
by  selecting  from  the  thirty  million  children  of  the  country 
those  having  superior  natural  ability  and  character,  by  train- 
ing them  and  giving  them  opportunity  to  do  the  work  for 
which  they  are  fit.  We  waste  the  mineral  resources  of  the 
country  and  the  fertility  of  the  soil,  but  our  most  scandalous 
waste  is  of  our  children,  most  of  all  of  those  who  might  be- 
come men  and  women  of  performance  and  of  genius. 

'  "Eugenics  may  become  the  most  important  of  all  applied 
sciences,  but  at  present  its  scientific  foundations  must  be  laid 
by  the  study  of  comparative  genetics,  on  the  one  side,  and  the 
study  of  human  conduct,  on  the  other.  There  is  more  im- 
mediate prospect  of  improving  our  civilization  than  our  germ- 
/  plasm.  It  is  easier  to  decrease  or  eliminate  typhoid  fever  by 
hygienic  measures  than  to  attain  racial  immunity,  although 
this  is  not  equally  the  case  for  tuberculosis  and  still  less  for 
cancer.  We  can  increase  to  anv  desired  extent  from  the 
existing  population  by  proper  selection  and  training  the 
number  of  scientific  workers  in  the  United  States.  The 
number  capable  of  exhibiting  genius  is  limited,  but  many  of 
them  are  lost  through  lack  of  opportunity.  It  is  our  business, 
it  should  be  our  principal  business,  to  improve  our  civiliza- 
tion by  giving  opportunity  to  those  who  are  fit,  while  at  the 
same  time  investigating  the  conditions  which  will  give  us  a 
better  race." 

Writers  on  sociology  have  shown  that  himian  progress  is 
largely  limited  and  determined  by  the  social  environment  and 
that  it  is  even  possible  for  social  progress  to  occur  in  spite  of 
biological  deterioration.  If  this  idea  is  correct,  one  argument 
for  control  of  human  matings  by  the  state  or  some  other 
central  agency  has  been  frequently  over-emphasized.  Racial 
progress  does  not  require  a  constantly  advancing  biological 


LIMITATIONS  OF  EUGENICS  305 

standard  in  the  individual.  As  individuals,  primitive  men 
were  probably  more  than  a  match  for  us  physically,  and  at 
least  our  equals  mentally.  As  regards  the  standard  of  the 
individual,  then,  the  race  has  not  progressed.  Civilization 
is  a  matter  of  collective  achievement;  it  is  not  a  biological 
inheritance  at  all,  but  a  cultural  one.  "  AVe  are  heirs  of  all 
the  ages  "  not  biologically,  but  only  culturally.  Standing  on 
the  shoulders  of  the  last  generation  we  see  farther  because 
we  are  higher  up,  not  because  we  are  taller. 

It  is  of  course  essential  that  the  racial  stock  be  kept  sound 
and  free  from  taint  of  disease  or  racial  poison,  but  granting 
this,  the  situation  is  not  so  alarming  as  some  persons  seem  to 
think.  For  the  normal  unperverted  instincts  of  the  average 
man  have  a  distinctly  eugenic  trend.  Cupid  is  a  safer  guide 
in  matrimony  than  a  licensing  board.  The  old  folks  always 
"  make  a  mess  of  it  "  when  they  interfere  in  the  match- 
making of  the  young  folks.  This  is  as  true  in  real  life  as  in 
literature.  Of  course  it  is  possible  for  young  folks  to  make 
mistakes  as  well  as  for  old  ones,  and  it  is  necessary  that  those 
older  persons  who  have  been  burned  by  the  fire,  or  have 
seen  others  suffer  in  like  fashion,  should  see  that  their  chil- 
dren do  not  fall  into  the  fire.  For  example,  civilization  has 
brought  into  being  many  perils  which  did  not  exist  in  a 
simpler  and  more  primitive  mode  of  living.  Of  these  the 
young  must  be  advised.  Implicit  trust  in  the  guidance  of  the 
instincts  will  in  a  civilized  community  lead  to  endless  trouble. 
Sexual  promiscuity  has  only  disastrous  consequences  among 
civilized  peoples  and  for  a  very  simple  reason,  the  certainty 
of  contamination  sooner  or  later  with  venereal  disease,  in 
particular  with  gonorrhoea  or  syphilis. 

It  is  probable  that  Polynesians,  before  the  advent  of 
Europeans,  were  free  from  these  diseases,  and  their  rather 
loose  sexual  relations,  as  viewed  by  our  standards,  had  no 
serious  racial  consequences.  But  with  the  advent  of  Euro- 
peans all  this  has  changed.  Continued  promiscuity  means  to 
them  now  racial  extermination,  as  it  does  among  Europeans. 
Sexual  purity  is  necessary  with  us,  not  merely  because  social 


306  GENETICS  AND  EUGENICS 

standards  demand  it,  but  because  avoidance  of  loathsome 
venereal  disease  is  impossible  otherwise. 

This  element  of  venereal  disease  has  frequently  been  an 
important  factor  in  determining  the  success  or  failure  of  race 
mixtures.  European  men  of  loose  morals  have  frequently 
introduced  venereal  disease  in  race  mixtures  with  native 
populations,  and  this  will  account  for  the  poor  results  ob- 
served in  many  racial  crosses.  When  this  element  is  absent, 
racial  crosses  of  Europeans  with  native  peoples  have  been 
observed  to  produce  offspring  of  complete  vigor  and  fertility. 
Racial  crossing  among  men,  as  among  domesticated  animals, 
is  biologically  beneficial  within  limits.  The  English  people 
were  originally  very  mixed  racially,  and  the  same  is  pre- 
eminently true  of  Americans  today.  This  mixture  of  elements 
not  too  dissimilar,  provided  the  social  heritage  is  not  unduly 
disturbed,  is  on  the  whole  beneficial.  It  results  in  increase 
of  vigor  and  energy  in  the  offspring,  together  with  an  in- 
crease of  variability,  physical  and  mental,  which  favors  social 
progress. 

It  is  certain  that  human  progress  depends  upon  two  sets  of 
agencies,  one  sociological  or  cultural,  the  other  biological. 
In  this  discussion  we  have  dealt  chiefiy  with  the  biological 
agencies.  Biologically  the  human  race  can  be  improved  only 
by  improvement  of  its  germ-plasm.  If  acquired  characters 
were  inherited,  we  might  hope  to  improve  the  human  race 
germinally  by  improving  the  environment.  If  as  seems 
more  probable  acquired  characters  are  not  to  any  consider- 
able extent  inherited,  then  environmental  agencies  affect  man 
chiefly  culturally,  not  biologically.  To  change  man  biologi- 
cally, to  make  a  different  sort  of  animal  of  him,  it  will  be 
necessary  to  act  through  heredity,  that  is  through  selection 
of  parents  for  the  next  generation. 

Leaving  aside  for  the  present  the  practical  difficulties  arid 
supposing  that  it  were  possible  to  manage  the  human  race 
like  a  stock  farm,  the  choice  of  parents  would  necessarily 
be  limited  by  the  material  available.  We  could  select  parents 
only  for  such  characteristics  as  the  human  race  today  pos- 


LIMITATIONS  OF  EUGENICS  307 

sesses.  We  could  not,  for  example,  breed  a  human  race  with 
wings,  however  desirable  such  a  characteristic  might  seem. 
We  are  limited  definitely  for  all  time  to  the  hand  type  of 
appendage.  But  there  are  different  types  and  sizes  of  hands 
among  human  beings  among  which  a  selection  might  be  nuide 
if  this  were  considered  desirable,  as  for  example  normal 
hands,  short-firigered  hands  (i.  e,,  brachydactyl),  hands  with 
a  reduced  number  of  fingers  {i.  e.,  syndactyl),  and  hands  with 
an  increased  number  of  fingers  {i.  e.,  polydactyl).  These 
several  types  of  hand  are  known  to  be  hereditary.  If  the 
unusual  types  were  superior  to  the  normal,  we  might  through 
heredity  make  them  replace  the  normal  in  the  race.  But  in 
reality,  the  normal  type  of  hand  seems  on  the  w^hole  to  be 
the  best  type,  and  so  we  have  no  desire  to  change  it.  The 
same  is  true  as  regards  most  human  traits  known  to  be  in- 
herited, whether  physical  or  intellectual.  Our  ideal  is  in 
I  general  the  normal.  There  are  certain  types  of  abnormality 
which  we  should  be  glad  to  see  become  less  frequent  in  occur- 
rence, as  for  example  albinism,  night  blindness,  color-blind- 
ness, and  haemophilia.  A  complete  control  of  heredity  would 
render  their  elimination  from  the  race  possible,  but  it  is 
doubtful  if  they  are  serious  enough  to  call  for  such  elmiina- 
tion,  even  if  human  matings  w^ere  wholly  controllable  by  a 
single  central  agency,  which  of  course  they  are  not.  For  in 
discriminating  against  persons  possessing  such  minor  defects 
as  these  we  should  be  in  danger  of  rejecting  some  of  our 
human  stock  which  is  best  in  regard  to  characteristics  of 
much  greater  consequence.  The  independent  inheritance  of 
traits  must  ever  be  kept  in  mind  in  deciding  who  are  desir- 
able and  who  undesirable  parents,  weakness  in  one  particular 
being  frequently  offset  by  unusual  strength  in  another.  Those 
undesirable  traits  which  are  inherited  in  the  simplest  way, 
as  Mendelian  characters,  are  not  likely  to  become  very  com- 
mon in  a  freely  intermarrying  population.  It  is  only  when 
society  becomes  stratified,  and  class  distinctions  arise  with 
castes  or  families  closely  intermarrying,  that  heredity  is  likely 
to  bring  Mendelian  recessive  defects  repeatedly  to  the  sur- 


308  GENETICS  AND  EUGENICS 

face.  Democracy  is  as  safe  a  remedy  against  such  evils  as 
state  controlled  marriages  would  be,  if  they  were  obtainable. 

The  most  important  inherited  traits  are  probably  those 
which  are  quantitatively  variable,  which  occur  in  a  graded 
series,  like  bodily  size  and  strength,  mental  power,  and  power 
of  resisting  disease.  In  regard  to  these,  excellence  is  a  matter 
of  degree  and  is  relative.  Further  no  particular  grade  breeds 
true.  Regression  toward  the  normal  is  the  universal  rule. 
If  society  could  be  managed  like  a  stock  farm,  then  it  would 
be  possible  to  change  the  normal  toward  which  regression 
occurs,  very  slowly  and  gradually,  as  for  example  in  mental 
power.  The  average  grade  of  intelligence  could  be  raised  by 
rigid  selection  long  continued.  Possibly  this  has  occurred  in 
the  evolution  of  existing  races  of  men.  If  so,  it  has  occurred 
unconsciously  and  through  natural  selection  and  probably 
more  from  the  struggle  of  one  cultural  group  with  another 
than  from  the  struggle  of  one  individual  with  another.  But 
the  modern  eugenic  ideal  is  to  make  a  conscious  selection  of 
parents  within  the  group  with  a  view  to  elevating  the  normal 
within  the  group,  a  thing  that  has  not  hitherto  been  at- 
tempted, unless  in  Sparta  for  the  breeding  of  soldiers. 

If  there  were  a  central  directing  agency  which  had  the 
power  as  well  as  the  wisdom  to  control  matings  within  the 
group,  something  could  undoubtedly  be  done  slowly  to  ele- 
vate the  general  average  of  bodily  vigor  or  innate  mental 
power  within  the  group.  This  could  be  done  most  rapidly 
by  polygamy  which  w^ould  permit  of  a  relatively  rigid  selec- 
tion of  sires;  less  rapidly  under  monogamy  by  a  selection  of 
parents  among  both  sexes,  the  offspring  to  be  cared  for  largely 
by  the  rest  of  the  community.  But  the  social  consequences 
of  either  of  these  methods  are  so  tremendous,  so  subversive 
are  they  of  individual  liberty,  that  no  modem  civilized  com- 
munity has  been  willing  to  contemplate  either  of  them.  The 
whole  movement  of  modem  times  is  in  an  opposite  direction. 
Practically  therefore,  we  are  limited  to  such  eugenic  measures 
as  the  individual  will  voluntarily  take  in  the  light  of  present 
knowledge  of  heredity.    It  will  do  no  good,  but  only  harm,  to 


EUGENICS  AND  THE  INDIVIDUAL  309 

magnify  such  knowledge  unduly,  or  to  conceal  its  present 
limitations.  We  should  extend  such  knowledge  as  rapidly 
as  possible,  but  not  legislate  until  we  are  very  sure  of  our 
ground. 

Every  young  person  of  sound  and  healthy  stock  should 
look  forward  to  marriage  and  family  life  as  the  comi)k'lion 
of  a  normal  career  and  incidentally  as  fulfilling  an  obligation 
which  he  owes  to  his  country  and  his  race.  Any  young  per- 
son who  for  any  reason  finds  himself  debarred  from  this  part 
in  life  should  fulfill  the  racial  obligation  vicariously  by  helj)- 
ing  to  care  for  and  to  educate  the  children  of  his  more 
fortunate  fellows. 


APPENDIX 


APPENDIX 

EXPERIMENTS  IN  PLANT-HYBRIDISATION  ' 

By  Gregor  Mendel 

{Read  at  the  Meetings  of  the  8th  February  and  8th  March,  1865.) 

INTRODUCTORY   REMARKS 

Experience  of  artificial  fertilisation,  such  as  is  effected  with  orna- 
mental plants  in  order  to  obtain  new  variations  in  colour,  has  led 
to  the  experiments  which  will  here  be  discussed.  The  striking' 
regularity  with  which  the  same  hybrid  forms  always  reajijieared 
whenever  fertilisation  took  place  between  the  same  species  induced 
further  experiments  to  be  undertaken,  the  object  of  which  was  to 
follow  up  the  developments  of  the  hybrids  in  their  progeny. 

To  this  object  numerous  careful  observers,  such  as  Kolreuter, 
Gartner,  Herbert,  Lecoq,  Wichura  and  others,  have  devoted  a  part 
of  their  lives  with  inexhaustible  perseverance.  Gartner  especially, 
in  his  work  "Die  Bastarderzeugung  im  Pflanzenreiche "  (The  Pro- 
duction of  Hybrids  in  the  Vegetable  Kingdom),  has  recorded  ver>' 
valuable  observations;  and  quite  recently  Wichura  pu})lished 
the  results  of  some  profound  investigations  into  the  hybrids  of  the 
Willow.  That,  so  far,  no  generally  applicable  law  governing  the 
formation  and  development  of  hybrids  has  been  successfully  fornui- 
lated  can  hardly  be  wondered  at  by  anyone  who  is  acquainted  with 
the  extent  of  the  task,  and  can  appreciate  the  difficulties  witli 
which  experiments  of  this  class  have  to  contend.  A  final  decision 
can  only  be  arrived  at  when  we  shall  have  before  us  the  results  of 
detailed  experiments  made  on  plants  belonging  to  the  most  diverse 
orders. 

Those  who  survey  the  work  done  in  this  department  will  arrive 
at  the  conviction  that  among  all  the  numerous  experiments  made, 
not  one  has  been  carried  out  to  such  an  extent  and  in  such  a  way  as 

1  This  translation  was  made  by  the  Royal  Horticultural  Society  of  London,  and 
is  reprinted,  by  permission  of  the  Council  of  the  Society,  with  footnotes  adde<i  and 
minor  changes  suggested  by  Professor  W.  Bateson,  enclosed  within  ( ].  The  original 
paper  was  published  in  the  V&rh.  naturj.  Ver.  in  Brunn,  Abhandlungen,  iv.  1805, 

which  appeared  in  1866. 

313 


314  APPENDIX 

to  make  it  possible  to  determine  the  number  of  different  forms 
under  which  the  offspring  of  hybrids  appear,  or  to  arrange  these 
forms  with  certainty  according  to  their  separate  generations,  or 
definitely  to  ascertain  their  statistical  relations.^ 

It  requires  indeed  some  courage  to  undertake  a  labour  of  such 
far-reaching  extent;  this  appears,  however,  to  be  the  only  right  way 
by  which  we  can  finally  reach  the  solution  of  a  question  the  impor- 
tance of  which  cannot  be  overestimated  in  connection  with  the 
history  of  the  evolution  of  organic  forms. 

The  paper  now  presented  records  the  results  of  such  a  detailed 
experiment.  This  experiment  was  practically  confined  to  a  small 
plant  group,  and  is  now,  after  eight  years'  pursuit,  concluded  in  all 
essentials.  Whether  the  plan  upon  which  the  separate  experiments 
were  conducted  and  carried  out  was  the  best  suited  to  attain  the 
desired  end  is  left  to  the  friendly  decision  of  the  reader. 

Selection  of  the  Experimental  Plants 

The  value  and  utility  of  any  experiment  are  determined  by  the 
fitness  of  the  material  to  the  purpose  for  which  it  is  used,  and  thus 
in  the  case  before  us  it  cannot  be  immaterial  what  plants  are 
subjected  to  experiment  and  in  what  manner  such  experiments 
are  conducted. 

The  selection  of  the  plant  group  which  shall  serve  for  experiments 
of  this  kind  must  be  made  with  all  possible  care  if  it  be  desired  to 
avoid  from  the  outset  every  risk  of  questionable  results. 

The  experimental  plants  must  necessarily  — 

1.  Possess  constant  differentiating  characters. 

2.  The  hybrids  of  such  plants  must,  during  the  flowering  period, 
be  protected  from  the  influence  of  aU  foreign  pollen,  or  be  easily 
capable  of  such  protection. 

The  hybrids  and  their  offspring  should  suffer  no  marked  disturb- 
ance in  their  fertility  in  the  successive  generations. 

Accidental  impregnation  by  foreign  pollen,  if  it  occurred  during 
the  experiments  and  were  not  recognized,  would  lead  to  entirely 
erroneous  conclusions.  Reduced  fertility  or  entire  sterility  of  cer- 
tain forms,  such  as  occurs  in  the  offspring  of  many  hybrids,  would 
render  the  experiments  very  difficult  or  entirely  frustrate  them.    In 

^  [It  is  to  the  clear  conception  of  these  three  primary  necessities  that  the  whole 
success  of  Mendel's  work  is  due.  So  far  as  I  know  this  conception  was  absolutely 
new  in  his  day.] 


APPENDIX  315 

order  to  discover  the  relations  in  which  the  hybrid  forms  stand 
towards  each  other  and  also  towards  their  progenitors  it  appears 
to  be  necessary  that  all  members  of  the  series  developed  in  each 
successive  generation  should  be,  without  exception,  subjected  to 
observation. 

At  the  very  outset  special  attention  was  devoted  to  the  Legu- 
minosae  on  account  of  their  peculiar  floral  structure.  Exjjeriments 
which  were  made  with  several  members  of  this  family  led  to  the 
result  that  the  genus  Pisum  was  found  to  possess  the  necessary 
qualifications. 

Some  thoroughly  distinct  forms  of  this  genus  possess  characters 
which  are  constant,  and  easily  and  certainly  recognizable,  and 
when  their  hybrids  are  mutually  crossed  they  yield  perfectly  fertile 
progeny.  Furthermore,  a  disturbance  through  foreign  pollen 
cannot  easily  occur,  since  the  fertilising  organs  are  closely  packed 
inside  the  keel  and  the  anther  bursts  within  the  bud,  so  that  the 
stigma  becomes  covered  with  pollen  even  before  the  flower  opens. 
This  circumstance  is  of  especial  importance.  As  additional  advan- 
tages worth  mentioning,  there  may  be  cited  the  easy  culture  of  these 
plants  in  the  open  ground  and  in  pots,  and  also  their  relatively  short 
period  of  growth.  Artificial  fertilisation  is  certainly  a  somewhat 
elaborate  process,  but  nearly  always  succeeds.  For  this  purpose 
the  bud  is  opened  before  it  is  perfectly  developed,  the  keel  is 
removed,  and  each  stamen  carefully  extracted  by  means  of  forceps, 
after  which  the  stigma  can  at  once  be  dusted  over  with  the  foreign 
pollen. 

In  all,  thirty -four  more  or  less  distinct  varieties  of  Peas  were 
obtained  from  several  seedsmen  and  subjected  to  a  two  years'  trial. 
In  the  case  of  one  variety  there  were  noticed,  among  a  larger  num- 
ber of  plants  all  alike,  a  few  forms  which  were  markedly  dift'erent. 
These,  however,  did  not  vary  in  the  following  year,  and  agreed 
entirely  with  another  variety  obtained  from  the  same  seedsman;  the 
seeds  were  therefore  doubtless  merely  accidentally  mixed.  All  the 
other  varieties  yielded  perfectly  constant  and  similar  oftspring;  at 
any  rate,  no  essential  difference  was  observed  during  two  trial  years. 
For  fertilisation  twenty-two  of  these  were  selected  and  cultivated 
during  the  whole  period  of  the  experiments.  They  remained 
constant  without  any  exception. 

Their  systematic  classification  is  difficult  and  uncertain.  If  we 
adopt  the  strictest  definition  of  a  species,  according  to  which  only 


316  APPENDIX 

those  individuals  belong  to  a  species  which  under  precisely  the 
same  circumstances  display  precisely  similar  characters,  no  two  of 
these  varieties  could  be  referred  to  one  species.  According  to  the 
opinion  of  experts,  however,  the  majority  belong  to  the  species 
Pisum  sativum;  while  the  rest  are  regarded  and  classed,  some  as 
sub-species  of  P.  sativum,  and  some  as  independent  species,  such  as 
P.  quadratum,  P.  saccharatum,  and  P.  umhellatum.  The  positions, 
however,  which  may  be  assigned  to  them  in  a  classificatory  system 
are  quite  immaterial  for  the  purposes  of  the  experiments  in  ques- 
tion. It  has  so  far  been  found  to  be  just  as  impossible  to  draw  a 
sharp  line  between  the  hybrids  of  species  and  varieties  as  between 
species  and  varieties  themselves. 

Division  and  Arrangement  of  the  Experiments 

If  two  plants  which  differ  constantly  in  one  or  several  characters 
be  crossed,  numerous  experiments  have  demonstrated  that  the 
common  characters  are  transmitted  unchanged  to  the  hybrids  and 
their  progeny;  but  each  pair  of  differentiating  characters,  on  the 
other  hand,  unite  in  the  hybrid  to  form  a  new  character,  which  in 
the  progeny  of  the  hybrid  is  usually  variable.  The  object  of  the 
experiment  was  to  observe  these  variations  in  the  case  of  each  pair 
of  differentiating  characters,  and  to  deduce  the  law  according  to 
which  they  appear  in  the  successive  generations.  The  experiment 
resolves  itself  therefore  into  just  as  many  separate  experiments  as 
there  are  constantly  differentiating  characters  presented  in  the 
experimental  plants. 

The  various  forms  of  Peas  selected  for  crossing  showed  differences 
in  the  length  and  colour  of  the  stem;  in  the  size  and  form  of  the 
leaves;  in  the  position,  colour,  and  size  of  the  flowers;  in  the  length 
of  the  flower  stalk;  in  the  colour,  form,  and  size  of  the  pods;  in  the 
form  and  size  of  the  seeds;  and  in  the  colour  of  the  seed- coats  and 
of  the  albumen  [cotyledons].  Some  of  the  characters  noted  do  not 
permit  of  a  sharp  and  certain  separation,  since  the  difference  is  of  a 
**  more  or  less  "  nature,  which  is  often  difficult  to  define.  Such 
characters  could  not  be  utilised  for  the  separate  experiments ;  these 
could  only  be  applied  to  characters  which  stand  out  clearly  and 
definitely  in  the  plants.  Lastly,  the  result  must  show  whether  they, 
in  their  entirety,  observe  a  regular  behaviour  in  their  hybrid  unions, 
and  whether  from  these  facts  any  conclusion  can  be  come  to  re- 
garding those  characters  which  possess  a  subordinate  significance 
in  the  type. 


APPENDIX  317 

The  characters  which  were  selected  for  experiment  relate: 

1.  To  the  difference  in  the  form  of  the  ripe  seeda.  These  are  either 
round  or  roundish,  the  depressions,  if  any,  occur  on  the  surface, 
being  always  only  shallow;  or  they  are  irregularly  angular  and 
deeply  wrinkled  (P.  quadratum). 

2.  To  the  difference  in  the  colour  of  the  seed  albumen  (endosperm) . ' 
The  albumen  of  the  ripe  seeds  is  either  pale  yellow,  l)right  yellow 
and  orange  coloured,  or  it  possesses  a  more  or  less  intense  green 
tint.  This  difference  of  colour  is  easily  seen  in  the  seeds  as  [  =  if] 
their  coats  are  transparent. 

3.  To  the  difference  in  the  colour  of  the  seed-coat  This  is  either 
white,  with  which  character  white  flowers  are  constantly  corre- 
lated; or  it  is  grey,  grey-brown,  leather-brown,  with  or  without 
violet  spotting,  in  which  case  the  colour  of  the  standards  is  violet, 
that  of  the  wings  purple,  and  the  stem  in  the  axils  of  the  leaves  is  of 
a  reddish  tint.  The  grey  seed-coats  become  dark  brown  in  boiling 
water. 

4.  To  the  difference  in  the  form  of  the  ripe  pods.  These  are 
either  simply  inflated,  not  contracted  in  places;  or  they  are  deeply 
constricted  between  the  seeds  and  more  or  less  wrinkled  (P. 
saccharatum) . 

5.  To  the  difference  in  the  colour  of  the  unripe  pods.  They  are 
either  light  to  dark  green,  or  vividly  yellow,  in  which  colouring  the 
stalks,  leaf- veins,  and  calyx  participate. ^ 

6.  To  the  difference  in  the  position  of  the  flowers.  They  are  either 
axial,  that  is,  distributed  along  the  main  stem;  or  they  are  ter- 
minal, that  is,  bunched  at  the  top  of  the  stem  and  arranged  almost 
in  a  false  umbel ;  in  this  case  the  upper  part  of  the  stem  is  more  or 
less  widened  in  section  (P.  umbellatum)  .^ 

7.  To  the  difference  in  the  length  of  the  stem.  The  length  of  the 
stem^  is  very  various  in  some  forms;   it  is,  however,  a  constant 

^  [Mendel  uses  the  terms  *'  albumen  "  and  "  endosperm  "  somewhat  l<x).sely  to 
denote  the  cotyledons,  containing  food-material,  within  the  seed.] 

^  One  species  possesses  a  beautifully  brownish-red  coloured  |)od,  which  when 
ripening  turns  to  violet  and  blue.  Trials  with  this  character  were  only  l>egun  last 
year.  [Of  these  further  experiments  it  seems  no  account  was  published.  Corrcns  has 
since  worked  with  such  a  variety.] 

^  [This  is  often  called  the  Mummy  Pea.  It  shows  slight  fasciatiou.  The  form 
I  know  has  white  standard  and  salmon-red  wings.] 

^  [In  my  account  of  these  experiments  {R.H.S.  Journal,  vol.  xxv.  p.  54)  I  mis- 
understood this  paragraph  and  took  '"  axis  "  to  mean  the  Jioral  axis,  instead  of  the 


318  APPENDIX 

character  for  each,  in  so  far  that  healthy  plants,  grown  in  the  same 
soil,  are  only  subject  to  unimportant  variations  in  this  character. 

In  experiments  with  this  character,  in  order  to  be  able  to  dis- 
criminate with  certainty,  the  long  axis  of  6  to  7  ft.  was  always 
crossed  with  the  short  one  of  |  ft.  to  1|  ft. 

Each  two  of  the  differentiating  characters  enumerated  above 
were  united  by  cross-fertilisation.    There  were  made  for  the 

1st  trial  60  fertilisations  on  15  plants. 

2nd     "  58  "  "  10 

3rd  "  35  "  "  10 

4th  "  40  "  "  10 

5th  "  23  "  "  5 

6th  "  34  "  "  10 

7th  "  37  "  "  10 

From  a  larger  number  of  plants  of  the  same  variety  only  the  most 
vigorous  were  chosen  for  fertilisation.  Weakly  plants  always  afford 
uncertain  results,  because  even  in  the  first  generation  of  hybrids, 
and  still  more  so  in  the  subsequent  ones,  many  of  the  offspring 
either  entirely  fail  to  flower  or  only  form  a  few  and  inferior  seeds. 

Furthermore,  in  aU  the  experiments  reciprocal  crossings  were 
effected  in  such  a  way  that  each  of  the  two  varieties  which  in  one 
set  of  fertilisation  served  as  seed-bearer  in  the  other  set  was  used 
as  the  pollen  plant. 

The  plants  were  grown  in  garden  beds,  a  few  also  in  pots,  and 
were  maintained  in  their  naturally  upright  position  by  means  of 
sticks,  branches  of  trees,  and  strings  stretched  between.  For  each 
experiment  a  number  of  pot  plants  were  placed  during  the  blooming 
period  in  a  greenhouse,  to  serve  as  control  plants  for  the  main 
experiment  in  the  open  as  regards  possible  disturbance  by  insects. 
Among  the  insects  ^  which  visit  Peas  the  beetle  Bruchus  pisi  might 
be  detrimental  to  the  experiments  should  it  appear  in  numbers. 
The  female  of  this  species  is  known  to  lay  the  eggs  in  the  flower, 
and  in  so  doing  opens  the  keel;  upon  the  tarsi  of  one  specimen, 
which  was  caught  in  a  flower,  some  pollen  grains  could  clearly  be 
seen  under  a  lens.    Mention  must  also  be  made  of  a  circumstance 

main  axis  of  the  plant.  The  unit  of  measurement,  being  indicated  in  the  original 
by  a  dash  ('),  I  carelessly  took  to  have  been  an  inch,  but  the  translation  here  given 
is  evidently  correct.] 

^  [It  is  somewhat  surprising  that  no  mention  is  made  of  Thrips,  which  swarm  in 
Pea  flowers.  I  had  come  to  the  conclusion  that  this  is  a  real  source  of  error  and  I  see 
Laxton  held  the  same  opinion. 1 


APPENDIX  319 

which  possibly  might  lead  to  the  introduction  of  foreign  pollen. 
It  occurs,  for  instance,  in  some  rare  cases  that  certain  parts  of  an 
otherwise  quite  normally  developed  flower  wither,  resulting  in  a 
partial  exposure  of  the  fertilising  organs.  A  defective  development 
of  the  keel  has  also  been  observed,  owing  to  which  the  stigma  and 
anthers  remained  partially  uncovered.^  It  also  sometimes  hap- 
pens that  the  pollen  does  not  reach  full  perfection.  In  this  e\'ent 
there  occurs  a  gradual  lengthening  of  the  pistil  during  the  bloom- 
ing period,  until  the  stigmatic  tip  protrudes  at  the  point  of  the  keel. 
This  remarkable  appearance  has  also  been  observed  in  hybrids  of 
Phaseolus  and  Lathyrus. 

The  risk  of  false  impregnation  by  foreign  pollen  is,  however,  a 
very  slight  one  with  Pisum,  and  is  quite  incapable  of  disturbing  the 
general  result.  Among  more  than  10,000  plants  which  were  care- 
fully examined  there  were  only  a  very  few  cases  where  an  indubi- 
table false  impregnation  had  occurred.  Since  in  the  greenliouse 
such  a  case  was  never  remarked,  it  may  well  be  supposed  that 
Bruchus  pisi,  and  possibly  also  the  described  abnormalities  in  the 
floral  structure,  were  to  blame. 

[Fi]  The  Forms  of  the  Hybrids  ^ 

Experiments  which  in  previous  years  were  made  with  ornamental 
plants  have  already  afforded  evidence  that  the  hybrids,  as  a  rule, 
are  not  exactly  intermediate  between  the  parental  species.  With 
some  of  the  more  striking  characters,  those,  for  instance,  which 
relate  to  the  form  and  size  of  the  leaves,  the  pubescence  of  the 
several  parts,  &c.,  the  intermediate,  indeed,  is  nearly  always  to  be 
seen;  in  other  cases,  however,  one  of  the  two  parental  characters 
is  so  preponderant  that  it  is  difficult,  or  quite  impossible,  to  detect 
the  other  in  the  hybrid. 

This  is  precisely  the  case  with  the  Pea  hybrids.  In  the  case  of 
each  of  the  seven  crosses  the  hybrid-character  resembles  ^  that  of 
one  of  the  parental  forms  so  closely  that  the  other  either  escapes 

1  [This  also  happens  m  Sweet  Peas.] 

2  [Mendel  throughout  speaks  of  his  cross-bred  Peas  as  "  hybrids,"  a  terra  whR-h 
many  restrict  to  the  offspring  of  two  distinct  species.    He,  as  he  explains,  hold  this 

to  be  only  a  question  of  degree.]  i    ,    •  j 

3  [Note  that  Mendel,  with  true  penetration,  avoids  speaking  of  the  hybrid- 
character  as  "  transmitted  "  by  either  parent,  thus  escapmg  the  error  pervaduig  the 
older  views  of  heredity.] 


320  APPENDIX 

observation  completely  or  cannot  be  detected  with  certainty.  This 
circumstance  is  of  great  importance  in  the  determination  and 
classification  of  the  forms  under  which  the  offspring  of  the  hybrids 
appear.  Henceforth  in  this  paper  those  characters  which  are  trans- 
mitted entire,  or  almost  unchanged  in  the  hybridisation,  and  there- 
fore in  themseh^es  constitute  the  characters  of  the  hybrid,  are 
termed  the  dominant,  and  those  which  become  latent  in  the  process 
recessive.  The  expression  "  recessive  "  has  been  chosen  because  the 
characters  thereby  designated  withdraw  or  entirely  disappear  in 
the  hybrids,  but  nevertheless  reappear  unchanged  in  their  progeny, 
as  will  be  demonstrated  later  on. 

It  was  furthermore  shown  by  the  whole  of  the  experiments  that  it 
is  perfectly  immaterial  whether  the  dominant  character  belongs  to 
the  seed-bearer  or  to  the  pollen-parent;  the  form  of  the  hybrid 
remains  identical  in  both  cases.  This  interesting  fact  was  also 
emphasised  by  Gartner,  w^th  the  remark  that  even  the  most 
practised  expert  is  not  in  a  position  to  determine  in  a  hybrid  which 
of  the  two  parental  species  was  the  seed  or  the  pollen  plant. ^ 

Of  the  differentiating  characters  which  were  used  in  the  experi- 
ments the  following  are  dominant: 

1.  The  round  or  roundish  form  of  the  seed  with  or  without 
shallow  depressions. 

2.  The  yellow  colouring  of  the  seed  albumen  [cotyledons]. 

3.  The  grey,  grey-bro^m,  or  leather-brown  colour  of  the  seed- 
coat,  in  association  with  violet- red  blossoms  and  reddish  spots  in 
the  leaf  axils. 

4.  The  simply  inflated  form  of  the  pod. 

5.  The  green  colouring  of  the  unripe  pod  in  association  with  the 
same  colour  in  the  stems,  the  leaf-veins  and  the  calyx. 

6.  The  distribution  of  the  flowers  along  the  stem. 

7.  The  greater  length  of  stem. 

With  regard  to  this  last  character  it  must  be  stated  that  the 
longer  of  the  two  parental  stems  is  usually  exceeded  by  the  hybrid, 
a  fact  which  is  possibly  only  attributable  to  the  greater  luxuriance 
which  appears  in  all  parts  of  plants  when  stems  of  very  different 
length  are  crossed.  Thus,  for  instance,  in  repeated  experiments, 
stems  of  1  ft.  and  6  ft.  in  length  yielded  without  exception  hybrids 
which  varied  in  length  between  6  ft.  and  7|  ft. 

1  [Gartner,  p.  223.] 


APPENDIX  3^Zi 

The  hybrid  seeds  in  the  experiments  with  seed-coat  are  often 
more  spotted,  and  the  spots  sometimes  coalesce  into  small  bluish- 
violet  patches.  The  spotting  also  frequently  appears  even  when  it 
is  absent  as  a  parental  character.^ 

The  hybrid  forms  of  the  seed-shape  and  of  the  al])umen  [colour] 
are  developed  immediately  after  the  artificial  fertilisation  by  the 
mere  influence  of  the  foreign  pollen.  They  can,  therefore,  be 
observed  even  in  the  first  year  of  experiment,  whilst  all  tlie  other 
characters  naturally  only  appear  in  the  following  year  in  such 
plants  as  have  been  raised  from  the  crossed  seed. 

[F2]  The  Generation  [bred]  from  the  Hybrids 

In  this  generation  there  reappear,  together  with  the  dominant 
characters,  also  the  recessive  ones  with  their  peculiarities  fully 
developed,  and  this  occurs  in  the  definitely  expressed  average  pro- 
portion of  three  to  one,  so  that  among  each  four  plants  of  this 
generation  three  display  the  dominant  character  and  one  the 
recessive.  This  relates  without  exception  to  all  the  characters 
which  were  investigated  in  the  experiments.  The  angular  wrinkled 
form  of  the  seed,  the  green  colour  of  the  albumen,  the  white  colour 
of  the  seed-coats  and  the  flowers,  the  constrictions  of  the  pods,  the 
yellow  colour  of  the  unripe  pod,  of  the  stalk,  of  the  calyx,  and  of 
the  leaf  venation,  the  umbel-like  form  of  the  inflorescence,  and  the 
dwarfed  stem,  all  reappear  in  the  numerical  proportion  given, 
without  any  essential  alteration.  Transitional  forms  were  not 
observed  in  any  experiment. 

Since  the  hybrids  resulting  from  reciprocal  crosses  are  formed 
alike  and  present  no  appreciable  difference  in  their  subsequent 
development,  consequently  the  results  [of  the  reciprocal  crosses] 
can  be  reckoned  together  in  each  experiment.  The  relative  num- 
bers which  were  obtained  for  each  pair  of  differentiating  characters 

are  as  follows: 

Expt.  1.  Form  of  seed.  —  From  253  hybrids  7,324  seeds  were 
obtained  in  the  second  trial  year.  Amon^;  them  were  5,474  round 
or  roundish  ones  and  1,850  angular  wrinkled  ones.  Therefrom  the 
ratio  2.96  to  1  is  deduced. 

Expt.  2.  Colour  of  albumen.  —  258  plants  yielded  8,023  seeds, 
6,022  yellow,  and  2,001  green;  their  ratio,  therefore,  is  as  3.01  to  1. 

1  [This  refers  to  the  coats  of  the  seeds  borne  by  Fi  plants.) 


322  APPENDIX 

In  these  two  experiments  each  pod  yielded  usually  both  kinds  of 
seeds.  In  well-developed  pods  which  contained  on  the  average  six 
to  nine  seeds,  it  often  happened  that  all  the  seeds  were  round 
(Expt.  1)  or  all  yellow  (Expt.  2);  on  the  other  hand  there  were 
never  observed  more  than  five  wrinkled  or  five  green  ones  in  one 
pod.  It  appears  to  make  no  difference  whether  the  pods  are 
developed  early  or  later  in  the  hybrid  or  whether  they  spring  from 
the  main  axis  or  from  a  lateral  one.  In  some  few  plants  only  a  few 
seeds  developed  in  the  first  formed  pods,  and  these  possessed  exclu- 
sively one  of  the  two  characters,  but  in  the  subsequently  developed 
pods  the  normal  proportions  were  maintained  nevertheless. 

As  in  separate  pods,  so  did  the  distribution  of  the  characters  vary 
in  separate  plants.  By  way  of  illustration  the  first  ten  individuals 
from  both  series  of  experiments  may  serve. 


Experiment  1. 

Experiment  2. 

Fonn  of  Seed. 

Color  of  Albumen 

Plants                Round 

Angular 

Yellow 

Green 

1 

45 

12 

25 

11 

2 

•    27 

8 

32 

7 

3 

24 

7 

14 

5 

4 

19 

10 

70 

27 

5 

32 

11 

24 

13 

6 

26 

6 

20 

6 

7 

88 

24 

32 

13 

8 

22 

10 

44 

9 

9 

28 

6 

50 

14 

10 

25 

7 

44 

18 

As  extremes  in  the  distribution  of  the  two  seed  characters  in  one 
plant,  there  were  observed  in  Expt.  1  an  instance  of  43  round  and 
only  2  angular,  and  another  of  14  round  and  15  angular  seeds.  In 
Expt.  2  there  was  a  case  of  32  yellow  and  only  1  green  seed,  but  also 
one  of  20  yellow  and  19  green. 

These  two  experiments  are  important  for  the  determination  of 
the  average  ratios,  because  with  a  smaller  number  of  experimental 
plants  they  show  that  very  considerable  fluctuations  may  occur. 
In  counting  the  seeds,  also,  especially  in  Expt.  2,  some  care  is 
requisite,  since  in  some  of  the  seeds  of  many  plants  the  green  colour 
of  the  albumen  is  less  developed,  and  at  first  may  be  easily  over- 
looked. The  cause  of  this  partial  disappearance  of  the  green 
colouring  has  no  connection  with  the  hybrid-character  of  the  plants, 
as  it  likewise  occurs  in  the  parental  variety.     This  peculiarity 


APPENDIX  3!23 

[bleaching]  is  also  confined  to  the  individual  and  is  not  inherited  by 
the  offspring.  In  luxuriant  plants  this  appearance  was  freciuently 
noted.  Seeds  which  are  damaged  by  insects  during  their  develop- 
ment often  vary  in  colour  and  form,  but,  with  a  little  i)ractice  in 
sorting,  errors  are  easily  avoided.  It  is  almost  sui)crfiuous  to  men- 
tion that  the  pods  must  remain  on  the  plants  until  they  are  thor- 
oughly ripened  and  have  become  dried,  since  it  is  only  then  that 
the  shape  and  colour  of  the  seed  are  fully  developed. 

Expt.  3.  Colour  of  the  seed-coats.  —  Among  929  plants  705  bore 
violet-red  flowers  and  grey-brown  seed-coats;  224  had  white  flowers 
and  white  seed-coats,  giving  the  proportion  3.15  to  1. 

Expt.  4.  Form  of  pods.  —  Of  1,181  plants  882  had  them  simply 
inflated,  and  in  299  they  were  constricted.  Resulting  ratio, 
2.95  to  1. 

Expt.  5.  Colour  of  the  unripe  pods.  —  The  number  of  trial 
plants  was  580,  of  which  428  had  green  pods  and  152  yellow  ones. 
Consequently  these  stand  in  the  ratio  2.82  to  1. 

Expt.  6.  Position  of  flowers.  —  Among  858  cases  651  had 
inflorescences  axial  and  207  terminal.    Ratio,  3.14  to  1. 

Expt.  7.  Length  of  stem.  —  Out  of  1,064  plants,  in  787  cases 
the  stem  was  long,  and  in  277  short.  Hence  a  mutual  ratio  of  2.84 
to  1.  In  this  experiment  the  dwarfed  plants  were  carefully  lifted 
and  transferred  to  a  special  bed.  This  precaution  was  necessary, 
as  otherwise  they  would  have  perished  through  being  overgrown 
by  their  tall  relatives.  Even  in  their  quite  young  state  they  can  be 
easily  picked  out  by  their  compact  growth  and  thick  dark-green 
foliage.^ 

If  now  the  results  of  the  whole  of  the  experiments  be  brought 
together,  there  is  found,  as  between  the  number  of  forms  with  the 
dominant  and  recessive  characters,  an  average  ratio  of  2.98  to  1 , 
or  3  to  1. 

The  dominant  character  can  have  here  a  double  signification  — 
viz.  that  of  a  parental  character,  or  a  hybrid-character.'-  In  which 
of  the  two  significations  it  appears  in  each  separate  case  can  only 
be  determined  by  the  following  generation.  As  a  parental  char- 
acter it  must  pass  over  unchanged  to  the  whole  of  the  ott'spriug;  as 

1  [This  is  true  also  of  the  dwarf  or  "  Cupid  "  Sweet  Peas.] 

2  [This  paragraph  presents  the  view  of  the  hybrid-character  as  something  inci- 
dental to  the  hybrid,  and  not  "  transmitted  "  to  it  —  a  true  and  fundamental 
conception  here  expressed  probably  for  the  first  time.) 


SM  APPENDIX 

a  hybrid-character,  on  the  other  hand,  it  must  maintain  the  same 
behaviour  as  in  the  first  generation  [F2]. 

[F3]  The  Second  Generation  [bred]  from  the  Hybrids 

Those  forms  which  in  the  first  generation  [F2]  exhibit  the  reces- 
'^  V  sive  character  do  not  further  vary  in  the  second  generation  [F3]  as 
regards  this  character;  they  remain  constant  in  their  offspring. 

It  is  otherwise  with  those  which  possess  the  dominant  character 
in  the  first  generation  [bred  from  the  hybrids].  Of  these  ^w;o- thirds 
yield  offspring  which  display  the  dominant  and  recessive  char- 
acters in  the  proportion  of  3  to  1,  and  thereby  show  exactly  the 
same  ratio  as  the  hybrid  forms,  while  only  one-third  remains  with 
the  dominant  character  constant. 

The  separate  experiments  yielded  the  following  results : 

Expt.  1.  Among  565  plants  which  were  raised  from  round  seeds 
of  the  first  generation,  193  yielded  round  seeds  only,  and  re- 
mained therefore  constant  in  this  character;  372,  however,  gave 
both  round  and  wrinkled  seeds,  in  the  proportion  of  3  to  1.  The 
number  of  the  hybrids,  therefore,  as  compared  with  the  constants 
is  1.93  to  1. 

Expt.  2.  Of  519  plants  which  were  raised  from  seeds  whose 
albumen  was  of  yellow  colour  in  the  first  generation,  166  yielded 
exclusively  yellow,  while  353  yielded  yellow  and  green  seeds  in  the 
proportion  of  3  to  1.  There  resulted,  therefore,  a  division  into 
hybrid  and  constant  forms  in  the  proportion  of  2.13  to  1. 

For  each  separate  trial  in  the  following  experiments  100  plants 
were  selected  which  displayed  the  dominant  character  in  the  first 
generation,  and  in  order  to  ascertain  the  significance  of  this,  ten 
seeds  of  each  were  cultivated. 

Expt.  3.  The  offspring  of  36  plants  yielded  exclusively  grey- 
brown  seed-coats,  while  of  the  offspring  of  64  plants  some  had 
grey -brown  and  some  had  white. 

Expt.  4.  The  offspring  of  29  plants  had  only  simply  inflated 
pods;  of  the  offspring  of  71,  on  the  other  hand,  some  had  inflated 
and  some  constricted. 

Expt.  5.  The  offspring  of  40  plants  had  only  green  pods;  of  the 
offspring  of  60  plants  some  had  green,  some  yellow  ones. 

Expt.  6.  The  offspring  of  33  plants  had  only  axial  flowers;  of 
the  offspring  of  67,  on  the  other  hand,  some  had  axial  and  some 
terminal  flowers. 


APPENDIX  3^5 

Expt.  7.  The  offspring  of  28  plants  inherited  the  long  axis,  and 
those  of  72  plants  some  the  long  and  some  the  short  axis. 

In  each  of  these  experiments  a  certain  number  of  the  plants  came 
constant  with  the  dominant  character.  For  the  determination  of 
the  proportion  in  which  the  separation  of  the  forms  with  the  con- 
stantly persistent  character  results,  the  two  first  experiments  are 
of  especial  importance,  since  in  these  a  larger  number  of  plants  can 
be  compared.  The  ratios  1.93  to  1  and  2.13  to  1  gave  together 
almost  exactly  the  average  ratio  of  2  to  1.  The  sixth  experiment 
gave  a  quite  concordant  result;  in  the  others  the  ratio  varies  more 
or  less,  as  was  only  to  be  expected  in  view  of  the  smaller  numl^er  of 
100  trial  plants.  Experiment  5,  which  shows  the  greatest  depar- 
ture, was  repeated,  and  then,  in  lieu  of  the  ratio  of  60  and  40,  tliat 
of  65  and  35  resulted.  The  average  ratio  of  2  to  1  appears,  thereforey 
as  fixed  with  certainty.  It  is  therefore  demonstrated  that,  of  those 
forms  which  possess  the  dominant  character  in  the  first  generation, 
two-thirds  have  the  hybrid-character,  while  one-third  remains 
constant  with  the  dominant  character. 

The  ratio  of  3  to  1,  in  accordance  with  which  the  distribution  of 
the  dominant  and  recessive  characters  results  in  the  first  genera- 
tion, resolves  itself  therefore  in  all  experiments  into  the  ratio  of 
2:1:1  if  the  dominant  character  be  differentiated  according  to  its 
significance  as  a  hybrid-character  or  as  a  parental  one.  Since  the 
members  of  the  first  generation  [F2]  spring  directly  from  the  seed 
of  the  hybrids  [Fi],  it  is  now  clear  that  the  hybrids  form  seeds  having 
one  or  other  of  the  two  differentiating  characters,  and  of  these  one-half 
develop  again  the  hybrid  form,  while  the  other  half  yield  plants  which 
remain  constant  and  receive  the  dominant  or  the  recessive  characters 
[respectively]  in  equal  numbers. 

The  Subsequent  Generations  [bred]  from  the  Hybrids 

The  proportions  in  which  the  descendants  of  the  hybrids  develop 
and  split  up  in  the  first  and  second  generations  presumably  hold 
good  for  all  subsequent  progeny.  Experiments  1  and  2  have 
already  been  carried  through  six  generations,  3  and  7  through  five, 
and  4,  5,  and  6  through  four,  these  experiments  being  continued 
from  the  third  generation  with  a  small  number  of  plants,  and  no 
departure  from  the  rule  has  been  perceptible.  The  offspring  of  the 
hybrids  separated  in  each  generation  in  the  ratio  of  2:1:1  into 
hybrids  and  constant  forms. 


326  APPENDIX 

If  A  be  taken  as  denoting  one  of  the  two  constant  characters,  for 
instance  the  dominant,  a,  the  recessive,  and  Aa  the  hybrid  form  in 
which  both  are  conjoined,  the  expression 

A  +  ^Aa  +  a 

shows  the  terms  in  the  series  for  the  progeny  of  the  hybrids  of  two 
differentiating  characters. 

The  observation  made  by  Gartner,  Kolreuter,  and  others,  that 
hybrids  are  incHned  to  revert  to  the  parental  forms,  is  also  con- 
firmed by  the  experiments  described.  It  is  seen  that  the  number  of 
the  hybrids  which  arise  from  one  fertilisation,  as  compared  with  the 
number  of  forms  which  become  constant,  and  their  progeny  from 
generation  to  generation,  is  continually  diminishing,  but  that 
nevertheless  they  could  not  entirely  disappear.  If  an  average 
equality  of  fertility  in  all  plants  in  all  generations  be  assumed,  and 
if,  furthermore,  each  hybrid  forms  seed  of  which  one-half  yields 
hybrids  again,  while  the  other  half  is  constant  to  both  characters 
in  equal  proportions,  the  ratio  of  numbers  for  the  offspring  in  each 
generation  is  seen  by  the  following  summary,  in  which  A  and  a 
denote  again  the  two  parental  characters,  and  Aa  the  hybrid  forms. 
For  brevity's  sake  it  may  be  assumed  that  each  plant  in  each 
generation  furnishes  only  4  seeds. 


Generation 

A 

Aa 

a 

1 

1 

2 

1 

2 

6 

4 

6 

3 

28 

8 

28 

4 

120 

16 

120 

5 

496 

32 

496 

n 

Ratios 

A 

:  Aa 

a 

1 

:     2 

1 

3 

:    2 

3 

7 

;     2 

7 

15 

:     2 

15 

31 

:     2 

31 

2"— 1 

:    2 

r-1 

In  the  ten.th  generation,  for  instance,  2"  —  1  =  1023.  There 
result,  therefore,  in  each  2,084  plants  which  arise  in  this  generation 
1,023  with  the  constant  dominant  character,  1,023  with  the  reces- 
sive character,  and  only  two  hybrids. 

The  Offspring  of  Hybrids  in  which  several  Differen- 
TL\TiNG  Characters  are  Associated 

In  the  experiments  above  described  plants  were  used  which 
differed  only  in  one  essential  character.^    The  next  task  consisted 

^  [This  statement  of  Mendel's  in  the  Hght  of  present  knowledge  is  open  to  some 
misconception.    Though  his  work  makes  it  evident  that  such  varieties  may  exist. 


/' 


VPPENblX  8*27 


in  ascertaining  whether  the  law  of  development  discovered  in  these 
applied  to  each  pair  of  differentiating  characters  when  several 
diverse  characters  are  united  in  the  hybrid  by  crossing.  As  regards 
the  form  of  the  hybrids  in  these  cases,  the  experiments  showed 
throughout  that  this  invariably  more  nearly  approaches  to  that  one 
of  the  two  parental  plants  which  possesses  the  greater  miin})er  of 
dominant  characters.  If,  for  instance,  the  seed  plant  has  a  short 
stem,  terminal  white  flowers,  and  simply  inflated  pods;  tlio  pollen 
plant,  on  the  other  hand,  a  long  stem,  violet-red  flowers  distributed 
along  the  stem,  and  constricted  pods;  the  hybrid  resembles  the 
seed  parent  only  in  the  form  of  the  pod;  in  the  other  characters  it 
agrees  with  the  pollen  parent.  Should  one  of  the  two  parental 
types  possess  only  dominant  characters,  then  the  hybrid  is  scarcely 
or  not  at  all  distinguishable  from  it. 

Two  experiments  were  made  with  a  considerable  number  of 
plants.  In  the  first  experiment  the  parental  plants  differed  in  the 
form  of  the  seed  and  in  the  colour  of  the  albumen;  in  the  second 
in  the  form  of  the  seed,  in  the  colour  of  the  albumen,  and  in  the 
colour  of  the  seed-coats.  Experiments  with  seed  characters  give 
the  result  in  the  simplest  and  most  certain  way. 

In  order  to  facilitate  study  of  the  data  in  these  experiments,  the 
different  characters  of  the  seed  plant  will  be  indicated  by  A,  B,  C, 
those  of  the  pollen  plant  by  a,  6,  c,  and  the  hybrid  forms  of  the 
characters  by  Aa,  Bb,  and  Cc. 

Expt.  1.  —  AB,  seed  parents;  ab,  pollen  parents; 

Ay  form  round;  a,  form  wrinkled; 

B,  albumen  yellow.  6,  albumen  green. 

The  fertilised  seeds  appeared  round  and  yellow  like  those  of  the 
seed  parents.  The  plants  raised  therefrom  yielded  seeds  of  four 
sorts,  which  frequently  presented  themselves  in  one  pod.  In  all, 
55Q  seeds  were  yielded  by  15  plants,  and  of  these  there  were: 

315  round  and  yellow, 
101  wrinkled  and  yellow, 
108  round  and  green, 
32  wrinkled  and  green. 

it  is  very  unlikely  that  Mendel  could  have  had  seven  pairs  of  varieties  such  that  the 
members  of  each  pair  differed  from  each  other  in  cnhj  one  considerable  character 
{wesentliches  Merkmal).  The  point  is  probably  of  little  theoretical  or  practical 
consequence,  but  a  rather  heavy  stress  is  thrown  on  "  wesentlichr  ] 


328  APPENDIX 

All  were  sown  the  following  year.  Eleven  of  the  round  yellow  seeds 
did  not  yield  plants,  and  three  plants  did  not  form  seeds.  Among 
the  rest: 

38  had  round  yellow  seeds AB 

65  round  yellow  and  green  seeds ABb 

60  round  yellow  and  wrinkled  yellow  seeds    .    .    ,    .  AaB 
138  round  yellow  and  green,  wrinkled  yellow  and 

green  seeds AaBb 

From  the  wrinkled  yellow  seeds  96  resulting  plants  bore  seed,  of 

which: 

28  had  only  wrinkled  yellow  seeds  aB 

68  wrinkled  yellow  and  green  seeds         aBb. 

From  108  round  green  seeds  102  resulting  plants  fruited,  of  which: 

35  had  only  round  green  seeds  Ab 

67  round  and  wrinkled  green  seeds  Aab. 

The  wrinkled  green  seeds  yielded  30  plants  which  bore  seeds  all  of 
like  character;  they  remained  constant  ab. 

The  offspring  of  the  hybrids  appeared  therefore  under  nine 
different  forms,  some  of  them  in  very  unequal  numbers.  When 
these  are  collected  and  co-ordinated  we  find : 

38  plants  with  the  sign  AB 

35  "  "  "  "  Ab 

28  "  "  "  "  aB 

30  "  "  "  "  ab 

65  "  "  "  "  ABb 

68  "  "  "  "  aBb 

60  "  "  "  "  AaB 

67  "  "  "  "  Aab 

138  "  "  "  "  AaBb. 

The  whole  of  the  forms  may  be  classed  into  three  essentially 
different  groups.  The  first  includes  those  with  the  signs  AB,  Aby 
aB,  and  ab :  they  possess  only  constant  characters  and  do  not  vary 
again  in  the  next  generation.  Each  of  these  forms  is  represented 
on  the  average  thirty-three  times.  The  second  group  includes  the 
signs  ABb,  aBb,  AaB,  Aab:  these  are  constant  in  one  character  and 
hybrid  in  another,  and  vary  in  the  next  generation  only  as  regards 
the  hybrid-character.  Each  of  these  appears  on  an  average  sixty- 
five  times.    The  form  AaBb  occurs  138  times:  it  is  hybrid  in  both 


APPENDIX  3>9 

characters,  and  behaves  exactly  as  do  the  hybrids  from  wliich  it  is 
derived. 

If  the  numbers  in  which  the  forms  belonging  to  these  classes 
appear  be  compared,  the  ratios  of  1,  2,  4  are  unmistakably  evident. 
The  numbers  32,  65,  138  present  very  fair  approximatioas  to  the 
ratio  numbers  of  33,  66,  132. 

The  developmental  series  consists,  therefore,  of  nine  classes,  of 
which  four  appear  therein  always  once  and  are  constant  in  })ot}i 
characters;  the  forms  AB,  ab,  resemble  the  parental  forms,  the 
two  other  present  combinations  between  the  conjoined  cliaracters 
Ay  a,  B,  b,  which  combinations  are  likewise  possibly  coiLstant. 
Four  classes  appear  always  twice,  and  are  constant  in  one  cliaracter 
and  hybrid  in  the  other.  One  class  appears  four  times,  and  is 
hybrid  in  both  characters.  Consequently  the  offspring  of  the 
hybrids,  if  two  kinds  of  differentiating  characters  are  coml)ined 
therein,  are  represented  by  the  expression 

AB-j-Ab-^aB-{-ab  +  ^ABb  +  2aBb  +  ^AaB  +  ^Aab  +  4^AaBb. 

This  expression  is  indisputably  a  combination  series  in  which  the 
two  expressions  for  the  characters  A  and  a,  B  and  b  are  combined. 
We  arrive  at  the  full  number  of  the  classes  of  the  series  by  the 
combination  of  the  expressions: 

A  4-  2Aa  +  a 
B  -\-2Bb  +  b. 
Expt.  2. 

ABC,  seed  parents;  abc,  pollen  parents; 

A,  form  round;  a,  form  wrinkled; 

B,  albumen  yellow;  6,  albumen  green; 

C,  seed-coat  grey-brown.  c,  seed-coat  white. 

This  experiment  was  made  in  precisely  the  same  way  as  the 
previous  one.  Among  all  the  experiments  it  demanded  the  most 
time  and  trouble.  From  24  hybrids  687  seeds  were  obtained  in  all : 
these  were  all  either  spotted,  grey-brown  or  grey-green,  round  or 
wrinkled.^  From  these  in  the  following  year  639  plants  fruited, 
and,  as  further  investigation  showed,  there  were  among  them: 

1  [Note  that  Mendel  does  not  state  the  cotyledon-colour  of  the  first  crosses  in 
this  case;  for  as  the  coats  were  thick,  it  could  not  have  been  seen  without  opening  or 
peeling  the  seeds.] 


330 


APPENDIX 

8  plants  ABC 

22] 

slants  ABCc 

45  plants  ABbCe 

14      ' 

'      ABc 

17 

u 

AbCc 

36 

u 

aBbCc 

9      ' 

'      AbC 

25 

u 

aBCc 

38 

u 

AaBCc 

11      ' 

'      Abe 

20 

u 

abCc 

40 

u 

AabCc 

8      ' 

'      aBC 

15 

a 

ABbC 

49 

u 

AaBbC 

10      ' 

'      aBc 

18 

u 

ABbc 

48 

u 

AaBbc 

10      ' 

'      abC 

19 

(I 

aBbC 

7      ' 

'      abc 

24 

u 

aBbc 

14 

u 

AaBC 

78 

u 

AaBbCi 

18 

u 

AaBc 

20 

u 

AabC 

16 

<< 

A  abc 

The  whole  expression  contains  27  terms.  Of  these  8  are  constant 
in  all  characters,  and  each  appears  on  the  average  10  times;  l!2  are 
constant  in  two  characters,  and  hj^brid  in  the  third;  each  appears 
on  the  average  19  times;  6  are  constant  in  one  character  and  hybrid 
in  the  other  two ;  each  appears  on  the  average  43  times.  One  form 
appears  78  times  and  is  hybrid  in  all  of  the  characters.  The  ratios 
10,  19,  43,  78  agree  so  closely  with  the  ratios  10,  20,  40,  80,  or  1,  2, 
4,  8,  that  this  last  undoubtedly  represents  the  true  value. 

The  development  of  the  hybrids  when  the  original  parents  differ 
in  three  characters  results  therefore  according  to  the  following 
expression : 

ABC  +  ABc  +  AhC  +  Abc  +  aBC  +  aBc  +  abC  +  abc 
+  2  ABCc  +  2  AbCc  +  2  aBCc  +  2  abCc  +  2  ABbC 
+  2  ABbc  +  2  aBbC  +  2  aBbc  +  2  AaBC  +  2  AaBc 
+  2  AabC  H-  2  Aabc  +  4  ABbCc  +  4  aj56Cc  +  4  ^a5Cc 
+  4  ^a6Cc  +  4  ^a56C  +  4  AaBbc  +  8  AaBbCc. 

Here  also  is  involved  a  combination  series  in  which  the  expres- 
sions for  the  characters  A  and  a,  B  and  6,  C  and  c,  are  united. 
The  expressions 

A  -{-  2Aa  +  a 
B  +  ^Bb  +  b 
■  C  +  2Cc  +  c 

give  all  the  classes  of  the  series.  The  constant  combinations  w^hich 
occur  therein  agree  with  all  combinations  which  are  possible 
between  the  characters  A,  B^  C,  a,  6,  c;  two  thereof,  ABC  and  abc, 
resemble  the  two  original  parental  stocks. 

In  addition,  further  experiments  were  made  with  a  smaller  num- 
ber of  experimental  plants  in  which  the  remaining  characters  by 


APPENDIX  381 

twos  and  threes  were  united  as  hybrids:  all  yielded  ai^proximately 
the  same  results.  There  is  therefore  no  douht  that  for  the  whole 
of  the  characters  involved  in  the  experiments  the  principle  applies 
that  the  offspring  of  the  hybrids  in  which  several  essentially  different 
characters  are  combined  exhibit  the  terms  of  a  series  of  combinations^ 
in  which  the  developmental  series  for  each  pair  of  differentiating 
characters  are  united.  It  is  demonstrated  at  the  same  time  that  the 
relation  of  each  pair  of  different  characters  in  hybrid  union  is  inde- 
pendent of  the  other  differences  in  the  two  original  parental  stocks. 

If  n  represents  the  number  of  the  differentiating  characters  in  the 
two  original  stocks,  3"  gives  the  number  of  terms  of  the  c-oinbina- 
tion  series,  4"  the  number  of  individuals  which  belong  to  the  series, 
and  2"  the  number  of  unions  which  remain  constant.  The  series 
therefore  contains,  if  the  original  stocks  differ  in  four  characters, 
3^  =  81  classes,  #  =  9.5Q  individuals,  and  2^  =  16  constant  forms; 
or,  which  is  the  same,  among  each  9,5^  offspring  of  the  hybrids 
there  are  81  different  combinations,  16  of  which  are  constant. 

All  constant  combinations  which  in  Peas  are  possible  by  the 
combination  of  the  said  seven  differentiating  characters  were 
actually  obtained  by  repeated  crossing.  Their  number  is  given  by 
2^  =  128.  Thereby  is  simultaneously  given  the  practical  y)ro()f 
that  the  constant  characters  which  appear  in  the  several  varieties  of  a 
group  of  plants  may  be  obtained  in  all  the  associations  which  are  pos- 
sible according  to  the  [mathematical]  laws  of  combinationy  by  means  of 
repeated  artificial  fertilisation. 

As  regards  the  flowering  time  of  the  hybrids,  the  experiments  are 
not  yet  concluded.  It  can,  however,  already  be  stated  that  the 
time  stands  almost  exactly  between  those  of  the  seed  and  pollen 
parents,  and  that  the  constitution  of  the  hybrids  with  respect  to 
this  character  probably  follows  the  rule  ascertained  in  the  case  of 
the  other  characters.  The  forms  which  are  selected  for  experiments 
of  this  class  must  have  a  difference  of  at  least  twenty  days  from  tlic 
middle  flowering  period  of  one  to  that  of  the  other;  furthermore, 
the  seeds  when  sown  must  all  be  placed  at  the  same  depth  in  tlie 
earth,  so  that  they  may  germinate  simultaneously.  Also,  during 
the  whole  flowering  period,  the  more  important  variations  in  tem- 
perature must  be  taken  into  account,  and  the  partial  hastening  or 
delaying  of  the  flowering  which  may  result  therefrom.  It  is  clear 
that  this  experiment  presents  many  difficulties  to  be  overcome  and 
necessitates  great  attention. 


332  APPENDIX 

If  we  endeavour  to  collate  in  a  brief  form  the  results  arrived  at, 
we  find  that  those  differentiating  characters,  which  admit  of  easy 
and  certain  recognition  in  the  experimental  plants,  all  behave 
exactly  alike  in  their  hybrid  associations.  The  offspring  of  the 
hybrids  of  each  pair  of  differentiating  characters  are,  one-half, 
hybrid  again,  while  the  other  half  are  constant  in  equal  proportions 
having  the  characters  of  the  seed  and  pollen  parents  respectively. 
If  several  differentiating  characters  are  combined  by  cross-fertili- 
sation in  a  hybrid,  the  resulting  offspring  form  the  terms  of  a  com- 
bination series  in  which  the  combination  series  for  each  pair  of 
differentiating  characters  are  united. 

The  uniformity  of  behaviour  shown  by  the  whole  of  the  char- 
acters submitted  to  experiment  permits,  and  fully  justifies,  the 
acceptance  of  the  principle  that  a  similar  relation  exists  in  the  other 
characters  which  appear  less  sharply  defined  in  plants,  and  there- 
fore could  not  be  included  in  the  separate  experiments.  An  experi- 
ment with  peduncles  of  different  lengths  gave  on  the  whole  a  fairly 
satisfactory  result,  although  the  differentiation  and  serial  arrange- 
ment of  the  forms  could  not  be  effected  with  that  certainty  which 
is  indispensable  for  correct  experiment. 

The  Reproductive  Cells  of  the  Hybrids 

The  results  of  the  previously  described  experiments  led  to  further 
experiments,  the  results  of  which  appear  fitted  to  afford  some  con- 
elusions  as  regards  the  composition  of ^ the  egg  and  pollen  cells  of 
hybrids.  An  important  clue  is  afforded  in  Pisum  by  the  circum- 
stance that  among  the  progeny  of  the  hybrids  constant  forms 
appear,  and  that  this  occurs^,  too,  in  respect  of  all  combinations  of 
the  associated  characters.  So  far  as  experience  goes,  we  find  it  in 
every  case  confirmed  that  constant  progeny  can  only  be  formed 
when  the  egg  cells  and  the  fertilising  pollen  are  of  like  character,  so 
that  both  are  provided  with  the  material  for  creating  quite  similar 
individuals,  as  is  the  case  with  the  normal  fertilisation  of  pure 
species.  We  must  therefore  regard  it  as  certain  that  exactly  similar 
factors  must  be  at  work  also  in  the  production  of  the  constant  forms 
in  the  hybrid  plants.  Since  the  various  constant  forms  are  pro- 
duced in  one  plant,  or  even  in  one  flower  of  a  plant,  the  conclusion 
appears  logical  that  in  the  ovaries  of  the  hybrids  there  are  formed 
as  many  sorts  of  egg  cells,  and  in  the  anthers  as  many  sorts  of 
pollen  cells,  as  there  are  possible  constant  combination  forms,  and 


APPENDIX  338 

that  these  egg  and  pollen  cells  agree  in  their  internal  composition 
with  those  of  the  separate  forms. 

In  point  of  fact  it  is  possible  to  demonstrate  theoretically  tliat 
this  hypothesis  would  fully  suffice  to  account  for  the  development 
of  the  hybrids  in  the  separate  generatioas,  if  we  miglit  ;it  the  sanic 
time  assume  that  the  various  kinds  of  egg  and  pollen  cells  were 
formed  in  the  hybrids  on  the  average  in  equal  numbers'.^' 

In  order  to  bring  these  assumptions  to  an  experimental  pr«M)f, 
the  following  experiments  were  designed.  Two  forms  which  were 
constantly  different  in  the  form  of  the  seed  and  the  colour  of  the 
albumen  were  united  by  fertilisation. 

If  the  differentiating  characters  are  again  indicated  as  A,  ii, 
a,  6,  we  have: 

AB,  seed  parent;  ah,  pollen  parent; 

A,  form  round;  a,  form  wrinkled; 

B,  albumen  yellow.  6,  albumen  green. 

The  artificially  fertilised  seeds  were  sown  together  with  several 
seeds  of  both  original  stocks,  and  the  most  vigorous  examples  were 
chosen  for  the  reciprocal  crossing.    There  were  fertilised : 

1.  The  hybrids  with  the  pollen  of  AB. 

2.  The  hybrids    "       "        "        "   ah. 

3.  AB  "       "        "        "   the  hybrids. 

4.  ah  "       "        "        "   the  hybrids. 

For  each  of  these  four  experiments  the  whole  of  the  flowers  on 
three  plants  were  fertilised.  If  the  above  theory  be  correct,  there 
must  be  developed  on  the  hybrids  egg  and  pollen  cells  of  the  forms 
AB,  Ah,  aB,  ah,  and  there  would  be  combined: 

1.  The  egg  cells  AB,  Ah,  aB,  ah  with  the  pollen  cells  AB. 

2.  The  egg  cells  AB,  Ah,  aB,  ah  with  the  pollen  cells  ah. 

3.  The  egg  cells  AB  with  the  pollen  cells  AB,  Ah,  aB,  ah. 

4.  The  egg  cells  ah  with  the  pollen  cells  AB,  Ah,  aB,  ah. 

From  each  of  these  experiments  there  could  then  result  only  the 
following  forms: 

1.  AB,  ABh,  AaB,  AaBh.        3.  AB,  ABh,  AaB,  AuBb. 

2.  AaBh,  Aah,  aBh,  ah.  4.  AaBh,  Aah,  aBh,  ah. 

1  [This  and  the  preceding  paragraph  contain  the  essence  of  the  Mendcliaa 
principles  of  heredity.] 


334  APPENDIX 

If,  furthermore,  the  several  forms  of  the  egg  and  pollen  cells  of 
the  hybrids  were  produced  on  an  average  in  equal  numbers,  then 
in  each  experiment  the  said  four  combinations  should  stand  in  the 
same  ratio  to  each  other.  A  perfect  agreement  in  the  numerical 
relations  was,  however,  not  to  be  expected,  since  in  each  fertilisa- 
tion, even  in  normal  cases,  some  egg  cells  remain  undeveloped  or 
subsequently  die,  and  many  even  of  the  well-formed  seeds  fail  to 
germinate  when  sown.  The  above  assumption  is  also  limited  in  so 
far  that,  while  it  demands  the  formation  of  an  equal  number  of  the 
various  sorts  of  egg  and  pollen  cells,  it  does  not  require  that  this 
should  apply  to  each  separate  hybrid  with  mathematical  exactness. 

The  first  and  second  experiments  had  primarily  the  object  of 
proving  the  composition  of  the  hybrid  egg  cells,  while  the  third  and 
fourth  experiments  were  to  decide  that  of  the  pollen  cells. ^  As  is 
shown  by  the  above  demonstration  the  first  and  third  experiments 
and  the  second  and  fourth  experiments  should  produce  precisely 
the  same  combinations,  and  even  in  the  second  year  the  result 
should  be  partially  visible  in  the  form  and  colour  of  the  artificially 
fertilised  seed.  In  the  first  and  third  experiments  the  dominant 
characters  of  form  and  colour,  A  and  B,  appear  in  each  union,  and 
are  also  partly  constant  and  partly  in  hybrid  union  with  the  reces- 
sive characters  a  and  b,  for  which  reason  they  must  impress  their 
peculiarity  upon  the  whole  of  the  seeds.  All  seeds  should  therefore 
appear  round  and  yellow,  if  the  theory  be  justified.  In  the  second 
and  fourth  experiments,  on  the  other  hand,  one  union  is  hybrid  in 
form  and  in  colour,  and  consequently  the  seeds  are  round  and 
yellow;  another  is  hybrid  in  form,  but  constant  in  the  recessive 
character  of  colour,  whence  the  seeds  are  round  and  green;  the 
third  is  constant  in  the  recessive  character  of  form  but  hybrid  in 
colour,  consequently  the  seeds  are  wrinkled  and  yellow;  the  fourth 
is  constant  in  both  recessive  characters,  so  that  the  seeds  are 
wrinkled  and  green.  In  both  these  experiments  there  were  conse- 
quently four  sorts  of  seed  to  be  expected  — viz.  round  and  yellow, 
round  and  green,  wrinkled  and  yellow,  wrinkled  and  green. 

The  crop  fulfilled  these  expectations  perfectly.  There  were 
obtained  in  the 

1st  Experiment,  98  exclusively  round  yellow  seeds; 
3rd  "  94  "  "  "  " 

^  [To  prove,  namely,  that  both  were  similarly  differentiated,  and  not  one  or 
other  only.] 


APPENDIX  3,3 


o 


In  the  2d  Experiment,  31  round  and  yellow,  2G  round  and  green, 
27  wrinkled  and  yellow,  26  wrinkled  and  green  seeds. 

In  the  4th  Experiment,  24  round  and  yell<nv,  25  round  and  green, 
22  wrinkled  and  yellow,  26  wrinkled  and  green  seeds. 

There  could  scarcely  be  now  any  doubt  of  the  success  of  the 
experiment;  the  next  generation  must  afford  the  iinal  jjroof. 
From  the  seed  sown  there  resulted  for  the  first  experiment  90 
plants,  and  for  the  third  87  plants  which  fruited:  these  yielded 
for  the 

1st  Exp.  3rd  Exp. 

20  25     round  yellow  seeds AB 

23  19     round  yellow  and  green  seeds ABh 

25  22     round  and  wrinkled  yellow  seeds AaB 

22  21     round  and  wrinkled  green  and  yellow  seeds   ....  AaBh 

In  the  second  and  fourth  experiments  the  round  and  yellow 
seeds  yielded  plants  with  round  and  wrinkled  yellow  and  green 
seeds,  AaBb. 

From  the  round  green  seeds,  plants  resulted  with  round  and 
wrinkled  green  seeds,  Aab. 

The  wrinkled  yellow  seeds  gave  plants  with  wrinkled  yellow  and 
green  seeds,  aBb. 

From  the  wrinkled  green  seeds  plants  were  raised  which  yielded 
again  only  wrinkled  and  green  seeds,  ab. 

Although  in  these  two  experiments  likewise  some  seeds  did  not 
germinate,  the  figures  arrived  at  already  in  the  previous  year  were 
not  affected  thereby,  since  each  kind  of  seed  gave  plants  which,  as 
regards  their  seed,  were  like  each  other  and  different  from  the 
others.    There  resulted  therefore  from  the 

2d  Exp.  4th  Exp. 

31  24  plants  of  the  form  AaBh 

26  25  "        "    "       "     '1«^ 

27  22  "        "    "       "     aBb 
26                     27                 "        "    "       "     ob 

In  all  the  experiments,  therefore,  there  appeared  all  the  forms 
which  the  proposed  theory  demands,  and  they  came  in  nearly 

equal  numbers. 

In  a  further  experiment  the  characters  of  flower-colour  and 
length  of  stem  were  experimented  upon,  and  selection  was  so  made 
that  in  the  third  year  of  the  experiment  each  character  ought  to 
appear  in  half  of  all  the  plants  if  the  above  theory  were  correct. 
A,  B,  a,  b  serve  again  as  indicating  the  various  characters. 


336  APPENDIX 

A,  violet-red  flowers.  a,  white  flowers. 

B,  axis  long.  6,  axis  short. 

The  form  Ab  was  fertilised  with  ab,  which  produced  the  hybrid 
Aab.  Furthermore,  aB  was  also  fertilised  with  ab,  whence  the 
hybrid  aBb.  In  the  second  year,  for  further  fertilisation,  the  hybrid 
Aab  was  used  as  seed  parent,  and  hybrid  aBb  as  pollen  parent. 

Seed  parent,  Aab.  Pollen  parent,  aBb. 

Possible  egg  cells,  Ab,ab.  Pollen  cells,  aB^ab. 

From  the  fertilisation  between  the  possible  egg  and  pollen  cells 
four  combinations  should  result,  viz., 

AaBb  +  aBb  +  Aab  +  ab. 

From  this  it  is  perceived  that,  according  to  the  above  theory,  in 
the  third  year  of  the  experiment  out  of  all  the  plants 

Half  should  have  violet-red  flowers  (Aa),  Classes  1,  3 

"          "          "     white  flowers  (a)   '  "        2,  4 

«          «          "     a  long  axis  (Bb)  "        1,2 

"          "          "a  short  axis  (6)  "        3,  4 

From  45  fertilisations  of  the  second  year  187  seeds  resulted,  of 
which  only  166  reached  the  flowering  stage  in  the  third  year. 
Among  these  the  separate  classes  appeared  in  the  numbers  fol- 
lowing: 


Class 

Color  of  flower 

Stem 

1 

violet-red 

long 

47  times 

2 

white 

long 

40      " 

3 

violet-red 

short 

38      " 

4 

white 

short 

41      " 

There  subsequently  appeared 

The  violet-red  flower-colour  (Aa)  in  85  plants. 
"    white  "  "        (a)     in  81       " 

"     long  stem  {Bb)  in  87      " 

"     short    "  (6)     in  79      " 

The  theory  adduced  is  therefore  satisfactorily  confirmed  in  this 
experiment  also. 

For  the  characters  of  form  of  pod,  colour  of  pod,  and  position  of 
flowers,  experiments  were  also  made  on  a  small  scale,  and  results 
obtained  in  perfect  agreement.  All  combinations  which  were  pos- 
sible through  the  union  of  the  differentiating  characters  duly 
appeared,  and  in  nearly  equal  numbers. 


APPENDIX  337 

Experimentally,  therefore,  the  theory  is  confirmed  that  the  pea 
hybrids  form  egg  and  pollen  cells  which,  in  their  constitution,  represent 
in  equal  numbers  all  constant  forms  which  result  from  the  combination 
of  the  characters  united  in  fertilisation . 

The  difference  of  the  forms  among  the  progeny  of  the  hybrids, 
as  well  as  the  respective  ratios  of  the  numbers  in  which  they  are 
observed,  find  a  sufficient  explanation  in  the  principle  above 
deduced.  The  simplest  case  is  afforded  by  the  developmental  series 
of  each  pair  of  differentiating  characters.  This  series  is  repre- 
sented by  the  expression  A  +  ^Aa  +  a,  in  which  A  and  a  signify 
the  forms  with  constant  differentiating  characters,  and  Aa  the 
hybrid  form  of  both.  It  includes  in  three  different  classes  four 
individuals.  In  the  formation  of  these,  pollen  and  egg  cells  of  the 
form  A  and  a  take  part  on  the  average  equally  in  the  fertilisation; 
hence  each  form  [occurs]  twice,  since  four  individuals  are  formed. 
There  participate  consequently  in  the  fertilisation 

The  pollen  cells  A  -\-  A  -{-  a  ■\-  a 
The  egg  cells  A  -\-  A  -\-  a  -]r  a. 

It  remains,  therefore,  purely  a  matter  of  chance  which  of  the  two 
sorts  of  pollen  will  become  united  with  each  separate  egg  cell. 
According,  however,  to  the  law  of  probability,  it  will  always  hap- 
pen, on  the  average  of  many  cases,  that  each  pollen  form,  A  and  a, 
will  unite  equally  often  with  each  egg  cell  form,  A  and  a,  conse- 
quently one  of  the  two  pollen  cells  A  in  the  fertilisation  will  meet 
with  the  egg  cell  A  and  the  other  with  an  egg  cell  a,  and  so  likewise 
one  pollen  cell  a  will  unite  with  an  egg  cell  A,  and  the  other  with 

egg  cell  a. 

Pollen  cells  A  A     a  a 

\         .\        i 

Egg  cells  A  A    a  a 

The  result  of  the  fertilisation  may  be  made  clear  by  putting  the 
signs  for  the  conjoined  egg  and  pollen  cells  in  the  form  of  fractions, 
those  for  the  pollen  cells  above  and  those  for  the  egg  cells  below  the 

line.    We  then  have 

A      A       a       a 

_    I  —  _i_  —  J- - . 

A^  a^  A^  a 

In  the  first  and  fourth  term  the  egg  and  pollen  cells  are  of  like  kind, 
consequently  the  product  of  their  union  must  be  constant,  viz.  A 


338  APPENDIX 

and  a;  in  the  second  and  third,  on  the  other  hand,  there  again 
results  a  union  of  the  two  differentiating  characters  of  the  stocks, 
consequently  the  forms  resulting  from  these  fertilisations  are 
identical  with  those  of  the  hybrid  from  which  they  sprang.  There 
occurs  accordingly  a  repeated  hyhridisaiion.  This  explains  the  strik- 
ing fact  that  the  hybrids  are  able  to  produce,  besides  the  two 

A  a 

parental  forms,  offspring  which  are  like  themselves;    —  and  — 

a  A 

both  give  the  same  union  Aa,  since,  as  already  remarked  above, 
it  makes  no  difference  in  the  result  of  fertilisation  to  which  of  the 
two  characters  the  pollen  or  egg  cells  belong.     We  may  write  then 

--  +  -  +  -4--  =  ^  +  2^a  +  a. 
A       a      A      a 

This  represents  the  average  result  of  the  self -fertilisation  of  the 
hybrids  when  two  differentiating  characters  are  united  in  them. 
In  individual  flowers  and  in  individual  plants,  however,  the  ratios 
in  which  the  forms  of  the  series  are  produced  may  suffer  not  in- 
considerable fluctuations.^  Apart  from  the  fact  that  the  numbers 
in  which  both  sorts  of  egg  cells  occur  in  the  seed  vessels  can  only  be 
regarded  as  equal  on  the  average,  it  remains  purely  a  matter  of 
chance  which  of  the  two  sorts  of  pollen  may  fertilise  each  separate 
eg,g  cell.  For  this  reason  the  separate  values  must  necessarily  be 
subject  to  fluctuations,  and  there  are  even  extreme  cases  possible, 
as  were  described  earlier  in  connection  with  the  experiments  on  the 
form  of  the  seed  and  the  colour  of  the  albumen.  The  true  ratios 
of  the  numbers  can  only  be  ascertained  by  an  average  deduced 
from  the  sum  of  as  many  single  values  as  possible ;  the  greater  the 
number,  the  more  are  merely  chance  effects  eliminated. 

The  developmental  series  for  hybrids  in  which  two  kinds  of 
differentiating  characters  are  united  contains,  among  sixteen 
individuals,  nine  different  forms,  viz., 

AB-]-Ah^  aB  +  ah-\-  2ABb  +  ^aBb  -f  2AaB  +  Mah  +  4AaBb. 

Between  the  differentiating  characters  of  the  original  stocks,  Aa 
and  Bby  four  constant  combinations  are  possible,  and  consequently 
the  hybrids  produce  the  corresponding  four  forms  of  egg  and  pollen 
cells  ABy  Aby  aB,  ab,  and  each  of  these  will  on  the  average  figure 

^  [Whether  segregation  by  such  units  is  more  than  purely  fortuitous  may  perhaps 
be  determined  by  seriation.] 


APPENDIX  339 

four  times  in  the  fertilisation,  since  sixteen  individuals  are  inrluded 
in  the  series.     Therefore  the  participators  in  the  fertilisation  are 

Pollen  cells  AB  +  AB  -{-  AB  -{-  AB  -f  Ab  -f  Ah  +  Ab  +  Ab 

+  aB  -{-  aB  -\- aB -\- aB  +  ab  +  ab  +  ab -\-  aL 

Egg  ceUs      AB  +  AB  +  AB  +  AB  +  Ab  +  Ab  +  Ab  +  Ab 

+  aB-\-aB-{-aB-{-aB-\-ab-^ab-\-abi-  ab. 

In  the  process  of  fertilisation  each  pollen  form  unites  on  an  average 
equally  often  with  each  egg  cell  form,  so  that  each  of  tlie  four  pollen 
cells  AB  unites  once  with  one  of  the  forms  of  egg  cell  Ali,  Ab,  aB, 
ab.  In  precisely  the  same  way  the  rest  of  the  pollen  cells  of  the 
forms  Ab,  aB,  ab  unite  with  all  the  other  egg  cells.  We  obtain 
therefore 

AB      AB      AB      AB      Ab       Ab      Ab      Ab 
AB       Ab       aB        ab       AB      Ab      aB       ab 

_L.  £:?.  4-  £^  4-  £^  _i_  £^  4_  _^    ,    ab        ab       ah 
AB      Ab      aB       ab       AB      Tb      '^i      ab' 
or 

AB  +  ABh  +  AaB  +  AaBb  +  ABb  +  Ab  +  AaBb  +  Aab  +  AaB 

+  AaBb  +  aB  +  aBb  +  AaBb  +  Aab  +  aBb  +  ab  =  AB 

-f-  ^6  +  aB  +  a6  +  '^ABb  +  ^aBb  +  '^AaB  +  2.4a6  +  A  AaBb} 

In  precisely  similar  fashion  is  the  developmental  series  of  hybrids 
exhibited  when  three  kinds  of 'differentiating  characters  are  con- 
joined in  them.  The  hybrids  form  eight  various  kinds  of  egg 
and  pollen  cells  —  ABC,  ABc,  AbC,  Abe,  aBC,  aBc,  abC\  abc  —  and 
each  pollen  form  unites  itself  again  on  the  average  once  with  eacli 
form  of  egg  cell. 

The  law  of  combination  of  different  characters,  which  governs 
the  development  of  the  hybrids,  finds  therefore  its  foundation  and 
explanation  in  the  principle  enunciated,  that  the  hybrids  j)r()duce 
egg  cells  and  pollen  cells  which  in  equal  numbers  represent  all  con- 
stant forms  which  result  from  the  combinations  of  the  chanicters 
brought  together  in  fertilisation. 

1  [In  the  original  the  sign  of  equality  (  =  )  is  here  represented  by  -f ,  evidently  a 
misprint.] 


340  APPENDIX 

Experiments  with  Hybrids  of  other  Species  of  Plants 

It  must  be  the  object  of  further  experiments  to  ascertain  whether 
the  law  of  development  discovered  for  Pisum  applies  also  to  the 
hybrids  of  other  plants.  To  this  end  several  experiments  were 
recently  commenced.  Two  minor  experiments  with  species  of 
Phaseolus  have  been  completed,  and  may  be  here  mentioned. 

An  experiment  with  Phaseolus  vulgaris  and  Phaseolus  nanus  gave 
results  in  perfect  agreement.  Ph.  nanus  had,  together  with  the 
dwarf  axis,  simply  inflated,  green  pods.  Ph.  vulgaris  had,  on  the 
other  hand,  an  axis  10  feet  to  12  feet  high,  and  yellow-coloured 
pods,  constricted  when  ripe.  The  ratios  of  the  numbers  in  which 
the  different  forms  appeared  in  the  separate  generations  were  the 
same  as  with  Pisum.  Also  the  development  of  the  constant  com- 
binations resulted  according  to  the  law  of  simple  combination  of 
characters,  exactly  as  in  the  case  of  Pisum.     There  were  obtained 


Constant 
combinations 

Axis 

Colour  of 
the  unripe  pods 

Form  of 
the  ripe  pods 

1 

2 

long 

u 

green 

u 

inflated 
constricted 

3 
4 

u 
u 

yellow 
a 

inflated 
constricted 

5 
6 

short 

green 

U 

inflated 
constricted 

7 
8 

a 
u 

yellow 

a 

inflated 
constricted 

The  green  colour  of  the  pod,  the  inflated  forms,  and  the  long  axis 
were,  as  in  Pisiim,  dominant  characters. 

Another  experiment  with  two  very  different  species  of  Phaseolus 
had  only  a  partial  result.  Phaseolus  nanus,  L.,  served  as  seed 
parent,  a  perfectly  constant  species,  with  white  flowers  in  short 
racemes  and  small  white  seeds  in  straight,  inflated,  smooth  pods; 
as  pollen  parent  was  used  Ph.  multiflorus,  W.,  with  tall  winding 
stem,  purple-red  flowers  in  very  long  racemes,  rough,  sickle-shaped 
crooked  pods,  and  large  seeds  which  bore  black  flecks  and  splashes 
on  a  peach-blood-red  ground. 

The  hybrids  had  the  greatest  similarity  to  the  pollen  parent,  but 
the  flowers  appeared  less  intensely  coloured.  Their  fertility  was 
very  limited;  from  seventeen  plants,  which  together  developed 
many  hundreds  of  flowers,  only  forty-nine  seeds  in  all  were  obtained. 
These  were  of  medium  size,  and  were  flecked  and  splashed  similarly 


APPENDIX  341 

to  those  of  Ph.  multiflorusy  while  the  ground  colour  was  not  niJi- 
terially  different.  The  next  year  forty-four  plants  were  raised 
from  these  seeds,  of  which  only  thirty-one  readied  the  flowering 
stage.  The  characters  of  Ph.  nanuSy  which  had  been  altogetluT 
latent  in  the  hybrids,  reappeared  in  various  combinations;  their 
ratio,  however,  with  relation  to  the  dominant  plants  was  neces- 
sarily very  fluctuating  owing  to  the  small  number  of  trial  jjlants. 
With  certain  characters,  as  in  those  of  the  axis  and  the  form  of  pod 
it  was,  however,  as  in  the  case  of  Pisum,  almost  exactly  1 :3. 

Insignificant  as  the  results  of  this  experiment  may  be  as  regards 
the  determination  of  the  relative  numbers  in  which  the  various 
forms  appeared,  it  presents,  on  the  other  hand,  the  phenomenon  of 
a  remarkable  change  of  colour  in  the  flowers  and  seed  of  the  hybrids. 
In  Pisum  it  is  known  that  the  characters  of  the  flower-  and  seed- 
colour  present  themselves  unchanged  in  the  first  and  further 
generations,  and  that  the  offspring  of  the  hybrids  display  exclu- 
sively the  one  or  the  other  of  the  characters  of  the  original  stocks. 
It  is  otherwise  in  the  experiment  we  are  considering.  The  white 
flowers  and  the  seed-colour  of  Ph.  nanus  appeared,  it  is  true,  at 
once  in  the  first  generation  [froTn  the  hybrids]  in  one  fairly  fertile 
example,  but  the  remaining  thirty  plants  developed  flower-colours 
which  were  of  various  grades  of  purple-red  to  pale  violet.  The 
colouring  of  the  seed-coat  was  no  less  varied  than  that  of  the 
flowers.  No  plant  could  rank  as  fully  fertile;  many  produced  no 
fruit  at  all;  others  only  yielded  fruits  from  the  flowers  last  pro- 
duced, which  did  not  ripen.  From  fifteen  plants  only  were  well- 
developed  seeds  obtained.  The  greatest  disposition  to  infertility 
was  seen  in  the  forms  with  preponderantly  red  flowers,  since  out  of 
sixteen  of  these  only  four  yielded  ripe  seed.  Three  of  these  had  a 
similar  seed  pattern  to  Ph.  multiflorus,  but  with  a  more  or  less  pale 
ground  colour;  the  fourth  plant  yielded  only  one  seed  of  plain 
brown  tint.  The  forms  with  preponderantly  violet-coloured 
flowers  had  dark  brown,  black-brown,  and  quite  black  seeds. 

The  experiment  was  continued  through  two  more  generations 
under  similar  unfavorable  circumstances,  since  even  among  the 
offspring  of  fairly  fertile  plants  there  came  again  some  which  were 
less  fertile  or  even  quite  sterile.  Other  flower-  and  seed-colours 
than  those  cited  did  not  subsequently  present  themselves.  The 
forms  which  in  the  first  generation  [bred  from  tlie  hybrids]  con- 
tained one  or  more  of  the  recessive  characters  remained,  as  regards 


342  APPENDIX 

these,  constant  without  exception.  Also  of  those  plants  which 
possessed  violet  flowers  and  brown  or  black  seed,  some  did  not  vary 
again  in  these  respects  in  the  next  generation;  the  majority,  how- 
ever, yielded,  together  with  offspring  exactly  like  themselves,  some 
which  displayed  white  flowers  and  white  seed-coats.  The  red 
flowering  plants  remained  so  slightly  fertile  that  nothing  can  be 
said  with  certainty  as  regards  their  further  development. 

Despite  the  many  disturbing  factors  with  which  the  observations 
had  to  contend,  it  is  nevertheless  seen  by  this  experiment  that  the 
development  of  the  hybrids,  with  regard  to  those  characters  which 
concern  the  form  of  the  plants,  follows  the  same  laws  as  in  Pisum. 
With  regard  to  the  colour  characters,  it  certainly  appears  difficult 
to  perceive  a  substantial  agreement.  Apart  from  the  fact  that  from 
the  union  of  a  white  and  a  purple-red  colouring  a  whole  series  of 
colours  results  [in  F2],  from  purple  to  pale  violet  and  white,  the 
circumstance  is  a  striking  one  that  among  thirty-one  flowering 
plants  only  one  received  the  recessive  character  of  the  white  colour, 
while  in  Pisum  this  occurs  on  the  average  in  every  fourth  plant. 

Even  these  enigmatical  results,  however,  might  probably  be 
explained  by  the  law  governing  Pisum  if  we  might  assume  that  the 
colour  of  the  flowers  and  seeds  of  Ph.  multiflorus  is  a  combination  of 
two  or  more  entirely  independent  colours,  which  individually  act 
like  any  other  constant  character  in  the  plant.  If  the  flower- 
colour  A  were  a  combination  of  the  individual  characters  Ai 
-|-  ^^2  +  •  •  •  which  produce  the  total  impression  of  a  purple  colora- 
tion, then  by  fertilisation  with  the  differentiating  character, 
white  colour,  a,  there  would  be  produced  the  hybrid  unions  Aia  -\- 
A^a  +  •  •  .  and  so  would  it  be  with  the  corresponding  colouring  of 
the  seed-coats.^  According  to  the  above  assumption,  each  of  these 
hybrid  colour  unions  would  be  independent,  and  would  conse- 
quently develop  quite  independently  from  the  others.  It  is  then 
easily  seen  that  from  the  combination  of  the  separate  develop- 
mental series  a  complete  colour-series  must  result.  If,  for  instance, 
A  =  ^1+  Ai,  then  the  hybrids  Aia  and  A^a  form  the  develop- 
mental series  — 

Ai  +  '^Aia  +  a,  ^2  +  ^A^a  +  a. 

1  [As  it  fails  to  take  account  of  factors  introduced  by  the  albino  this  represen- 
tation is  imperfect.  It  is  however  interesting  to  know  that  Mendel  realized  the  fact 
of  the  existence  of  compound  characters,  and  that  the  rarity  of  the  white  recessives 
was  a  consequence  of  this  resolution.] 


APPENDIX  343 

The  members  of  this  series  can  enter  into  nine  different  combina- 
tions, and  each  of  these  denotes  another  colour  — 


1  A,At 

2  AiaA2 

1   yl2« 

2  AiA^a 

4  AiaA^a 

2  A2aa 

1  Aia 

2  Aiaa 

1  aa. 

The  figures  prescribed  for  the  separate  combinations  also  indicate 
how  many  plants  with  the  corresponding  colouring  ijelong  to  the 
series.  Since  the  total  is  sixteen,  the  whole  of  the  colours  are  on  the 
average  distributed  over  each  sixteen  plants,  but,  as  the  series  itself 
indicates,  in  unequal  proportions. 

Should  the  colour  development  really  happen  in  this  way,  we 
could  offer  an  explanation  of  the  case  above  described,  viz.  that  the 
white  flowers  and  seed-coat  colour  only  appeared  once  among 
thirty-one  plants  of  the  first  generation.  This  colouring  aj)pears 
only  once  in  the  series,  and  could  therefore  also  only  be  developed 
once  in  the  average  in  each  sixteen,  and  with  three  colour  char- 
acters only  once  even  in  sixty -four  plants. 

It  must,  nevertheless,  not  be  forgotten  that  the  explanation  here 
attempted  is  based  on  a  mere  hypothesis,  only  supported  by  the 
very  imperfect  result  of  the  experiment  just  described.  It  would, 
however,  be  well  worth  while  to  follow  up  the  development  of 
colour  in  hybrids  by  similar  experiments,  since  it  is  probable  that 
in  this  way  we  might  learn  the  significance  of  the  extraordinary 
variety  in  the  colouring  of  our  ornamental  flowers. 

So  far,  little  at  present  is  known  with  certainty  beyond  the  fact 
that  the  colour  of  the  flowers  in  most  ornamental  plants  is  an 
extremely  variable  character.  The  opinion  has  often  been  ex- 
pressed that  the  stability  of  the  species  is  greatly  disturbed  or 
entirely  upset  by  cultivation,  and  consequently  there  is  an  inclina- 
tion to  regard  the  development  of  cultivated  forms  as  a  matter  of 
chance  devoid  of  rules;  the  colouring  of  ornamental  plants  is  indeed 
usually  cited  as  an  example  of  great  instability.  It  is,  however,  not 
clear  why  the  simple  transference  into  garden  soil  should  result  in 
such  a  thorough  and  persistent  revolution  in  the  plant  organism. 
No  one  will  seriously  maintain  that  in  the  open  country  the 
development  of  plants  is  ruled  by  other  laws  than  in  the  garden 
bed.  Here,  as  there,  changes  of  type  must  take  place  if  the  condi- 
tions of  life  be  altered,  and  the  species  possesses  the  cai)acit>'  of 
fitting  itself  to  its  new  environment.    It  is  willingly  granted  that 


344  APPENDIX 

by  cultivation  the  origination  of  new  varieties  is  favoured,  and  that 
by  man's  labour  many  varieties  are  acquired  which,  under  natural 
conditions,  would  be  lost;  but  nothing  justifies  the  assumption 
that  the  tendency  to  the  formation  of  varieties  is  so  extraordinarily 
increased  that  the  species  speedily  lose  all  stability,  and  their 
offspring  diverge  into  an  endless  series  of  extremely  variable  forms. 
Were  the  change  in  the  conditions  the  sole  cause  of  variability  we 
might  expect  that  those  cultivated  plants  which  are  grown  for 
centuries  under  almost  identical  conditions  would  again  attain 
constancy.  That,  as  is  well  known,  is  not  the  case,  since  it  is  pre- 
cisely under  such  circumstances  that  not  only  the  most  varied  but 
also  the  most  variable  forms  are  found.  It  is  only  the  Leguminosaey 
like  Pisum,  Phaseolus,^  Lens,  whose  organs  of  fertilisation  are  pro- 
tected by  the  keel,  which  constitute  a  noteworthy  exception.  Even 
here  there  have  arisen  numerous  varieties  during  a  cultural  period 
of  more  than  1000  years  under  most  various  conditions;  these 
maintain,  however,  under  unchanging  environments  a  stability  as 
great  as  that  of  species  growing  wild. 

It  is  more  than  probable  that  as  regards  the  variability  of  culti- 
vated plants  there  exists  a  factor  which  so  far  has  received  little 
attention.  Various  experiments  force  us  to  the  conclusion  that  our 
cultivated  plants,  with  few  exceptions,  are  members  of  various 
hybrid  series,  whose  further  development  in  conformity  with  law  is 
varied  and  interrupted  by  frequent  crossings  inter  se.  The  circum- 
stance must  not  be  overlooked  that  cultivated  plants  are  mostly 
grown  in  great  numbers  and  close  together,  affording  the  most 
favourable  conditions  for  reciprocal  fertilisation  between  the  varie- 
ties present  and  the  species  itself.  The  probability  of  this  is  sup- 
ported by  the  fact  that  among  the  great  array  of  variable  forms 
solitary  examples  are  always  found,  which  in  one  character  or 
another  remain  constant,  if  only  foreign  influence  be  carefully 
excluded.  These  forms  behave  precisely  as  do  those  which  are 
known  to  be  members  of  the  compound  hybrid  series.  Also  with 
the  most  susceptible  of  all  characters,  that  of  colour,  it  cannot 
escape  the  careful  observer  that  in  the  separate  forms  the  inclina- 
tion to  vary  is  displayed  in  very  different  degrees.  Among  plants 
which  arise  from  one  spontaneous  fertilisation  there  are  often  some 
whose  offspring  vary  widely  in  the  constitution  and  arrangement 
of  the  colours,  while  that  of  others  shows  little  deviation,  and 

^  [Phaseolus  nevertheless  is  insect-fertilised.] 


APPENDIX  345 

among  a  greater  number  solitary  examples  occur  which  transmit 
the  colour  of  the  flowers  unchanged  to  their  offspring.  The  culti- 
vated species  of  Dianthus  afford  an  instructive  exami)l('  of  this. 
A  white-flowered  example  of  Dianthus  caryophyllii.s,  which  itself 
was  derived  from  a  white-flowered  variety,  was  shut  up  during  its 
blooming  period  in  a  greenhouse;  the  numerous  seeds  obtained 
therefrom  yielded  plants  entirely  white-flowered  like  itself.  A 
similar  result  was  obtained  from  a  sub-species,  with  red  flowers 
somewhat  flushed  w4th  violet,  and  one  w4th  flowers  white,  striped 
with  red.  Many  others,  on  the  other  hand,  which  were  similarly 
protected,  yielded  progeny  which  were  more  or  less  variously 
coloured  and  marked. 

Whoever  studies  the  coloration  which  results.  In  ornamental 
plants,  from  similar  fertilisation,  can  hardly  escape  the  conviction 
that  here  also  the  development  follows  a  definite  law,  which 
possibly  finds  its  expression  in  the  combination  of  several  inde- 
pendent colour  characters. 

Concluding  Remarks 

It  can  hardly  fail  to  be  of  interest  to  compare  the  observations 
made  regarding  Pisum  with  the  results  arrived  at  by  the  two 
authorities  in  this  branch  of  knowledge,  Kolreuter  and  Gartner, 
in  their  investigations.  According  to  the  opinion  of  both,  the 
hybrids  in  outward  appearance  present  either  a  form  intermediate 
betw^een  the  original  species,  or  they  closely  resemble  either  the 
one  or  the  other  type,  and  sometimes  can  hardly  be  discriminated 
from  it.  From  their  seeds  usually  arise,  if  the  fertilisation  was 
effected  by  their  own  pollen,  various  forms  which  differ  from  the 
normal  type.  As  a  rule,  the  majority  of  individuals  obtained  In- 
one  fertilisation  maintain  the  hybrid  form,  while  some  few  others 
come  more  like  the  seed  parent,  and  one  or  other  individual 
approaches  the  pollen  parent.  This,  however,  is  not  the  case  with 
all  hybrids  without  exception.  Sometimes  the  offsi)ring  have 
more  nearly  approached,  some  the  one  and  some  the  other  of  the 
two  original  stocks,  or  they  all  incline  more  to  one  or  the  other 
side;  while  in  other  cases  they  remain  perfectly  like  the  hybrid  and 
continue  constant  in  their  offspring.  The  hybrids  of  varieties 
behave  like  hybrids  of  species,  but  they  possess  greater  variability 
of  form  and  a  more  pronounced  tendency  to  revert  to  the  original 
types. 


346  APPENDIX 

With  regard  to  the  form  of  the  hybrids  and  their  development, 
as  a  rule  an  agreement  with  the  observations  made  in  Pisum  is 
unmistakable.  It  is  otherwise  with  the  exceptional  cases  cited. 
Gartner  confesses  even  that  the  exact  determination  whether  a 
form  bears  a  greater  resemblance  to  one  or  to  the  other  of  the  two 
original  species  often  involved  great  difficulty,  so  much  depending 
upon  the  subjective  point  of  view  of  the  observer.  Another  cir- 
cumstance could,  however,  contribute  to  render  the  results  fluctu- 
ating and  uncertain,  despite  the  most  careful  observation  and 
differentiation.  For  the  experiments,  plants  were  mostly  used 
which  rank  as  good  species  and  are  differentiated  by  a  large  number 
of  characters.  In  addition  to  the  sharply  defined  characters,  where 
it  is  a  question  of  greater  or  less  similarity,  those  characters  must 
also  be  taken  into  account  which  are  often  difficult  to  define  in 
w^ords,  but  yet  suffice,  as  every  plant  specialist  knows,  to  give  the 
forms  a  peculiar  appearance.  If  it  be  accepted  that  the  develop- 
ment of  hybrids  follows  the  law  which  is  valid  for  Pisum,  the  series 
in  each  separate  experiment  must  contain  very  many  forms,  since 
the  number  of  the  terms,  as  is  known,  increases,  w^ith  the  number 
of  the  differentiating  characters,  as  the  powers  of  three.  With  a 
relatively  small  number  of  experimental  plants  the  result  therefore 
could  only  be  approximately  right,  and  in  single  cases  might 
fluctuate  considerably.  If,  for  instance,  the  two  original  stocks 
differ  in  seven  characters,  and  100  or  200  plants  were  raised  from 
the  seeds  of  their  hybrids  to  determine  the  grade  of  relationship  of 
the  offspring,  we  can  easily  see  how  uncertain  the  decision  must 
become,  since  for  seven  differentiating  characters  the  combination 
series  contain  16,384  individuals  under  2187  various  forms;  now 
one  and  then  another  relationship  could  assert  its  predominance, 
just  according  as  chance  presented  this  or  that  form  to  the  observer 
in  a  majority  of  cases. 

If,  furthermore,  there  appear  among  the  differentiating  char- 
acters at  the  same  time  dominant  characters,  which  are  transmitted 
entire  or  nearly  unchanged  to  the  hybrids,  then  in  the  terms  of  the 
developmental  series  that  one  of  the  two  original  parents  which 
possesses  the  majority  of  dominant  characters  must  always  be  pre- 
dominant. In  the  experiment  described  relative  to  Pisum,  in 
which  three  kinds  of  differentiating  characters  were  concerned,  all 
the  dominant  characters  belonged  to  the  seed  parent.  Although 
the  terms  of  the  series  in  their  internal  composition  approach  both 


APPENDIX  347 

original  parents  equally,  yet  in  this  experiment  the  type  of  the  seed 
parent  obtained  so  great  a  preponderance  that  out  of  each  sixty- 
four  plants  of  the  first  generation  fifty-four  exactly  reseniljled  it,  or 
only  differed  in  one  character.  It  is  seen  how  rash  it  must  be  under 
such  circumstances  to  draw  from  the  external  reseml)lances  of 
hybrids  conclusions  as  to  their  internal  nature. 

Gartner  mentions  that  in  those  cases  w^here  the  develoi)incnt  was 
regular,  among  the  offspring  of  the  hybrids,  the  two  original  si)ecies 
w^ere  not  reproduced,  but  only  a  few  individuals  which  approaclied 
them.  With  very  extended  developmental  series  it  could  not  in 
fact  be  otherwise.  For  seven  differentiating  characters,  for 
instance,  among  more  than  16,000  individuals  —  offspring  of  the 
hybrids  —  each  of  the  two  original  species  w^ould  occur  only  once. 
It  is  therefore  hardty  possible  that  these  should  appear  at  all  among 
a  small  number  of  experimental  plants;  with  some  probability, 
however,  we  might  reckon  upon  the  appearance  in  the  series  of  a 
few  forms  which  approach  them. 

We  meet  with  a.n  essential  difference  in  those  hybrids  which  re- 
main constant  in  their  progeny  and  propagate  themselves  as  truly  as 
the  pure  species.  According  to  Gartner,  to  this  class  belong  the 
remarkably  fertile  hybrids^  Aquilegia  atropurpurea  canadensis, 
Lavatera  pseudolbia  thuringiaca,  Geum  urbano-rivale,  and  some 
Dianthus  hybrids;  and,  according  to  Wichura,  the  hybrids  of  the 
Willow  family.  For  the  history  of  the  evolution  of  plants  this  cir- 
cumstance is  of  special  importance,  since  constant  hybrids  acquire 
the  status  of  new  species.  The  correctness  of  the  facts  is  guaran- 
teed by  eminent  observers,  and  cannot  be  doubted.  Gartner  had 
an  opportunity  of  following  up  Dianthus  Armeria  deltoides  to  the 
tenth  generation,  since  it  regularly  propagated  itself  in  the  garden. 

With  Pisum  it  was  shown  by  experiment  that  the  hybrids  form 
egg  and  pollen  cells  of  different  kinds,  and  that  herein  lies  the  reason 
of  the  variability  of  their  offspring.  In  other  hybritls,  likewise, 
whose  offspring  behave  similarly  we  may  assume  a  like  cause;  for 
those,  on  the  other  hand,  which  remain  constant,  the  a-ssum})tion 
appears  justifiable  that  their  reproductive  cells  are  all  alike  and 
agree  with  the  foundation-cell  [fertilised  ovum]  of  the  hybrid,  lii 
the  opinion  of  renowned  physiologists,  for  the  purpose  of  proi)aga- 
tion  one  pollen  cell  and  one  Qgg  cell  unite  in  Phanerogams  ^  into  a 

1  In  Pisum  it  is  placed  beyond  doubt  that  for  the  formation  of  the  new  embryo  a 
perfect  union  of  the  elements  of  both  reproductive  cells  must  take  place.     How 


348 


APPENDIX 


single  cell,  which  is  capable  by  assimilation  and  formation  of  new 
cells  to  become  an  independent  organism.  This  development  fol- 
lows a  constant  law,  which  is  founded  on  the  material  composition 
and  arrangement  of  the  elements  which  meet  in  the  cell  in  a  vivify- 
ing miion.  If  the  reproductive  cells  be  of  the  same  kind  and  agree 
with  the  foundation  cell  [fertilised  ovum]  of  the  mother  plant,  then 
the  development  of  the  new  individual  will  follow  the  same  law 
which  rules  the  mother  plant.  If  it  chance  that  an  egg  cell  unites 
with  a  dissimilar  pollen  cell,  we  must  then  assume  that  between 
those  elements  of  both  cells,  which  determine  opposite  characters, 
some  sort  of  compromise  is  effected.  The  resulting  compound  cell 
becomes  the  foundation  of  the  hybrid  organism,  the  development 
of  which  necessarily  follows  a  different  scheme  from  that  obtaining 
in  each  of  the  two  original  species.  If  the  compromise  be  taken  to 
be  a  complete  one,  in  the  sense,  namely,  that  the  hybrid  embryo 
is  formed  from  two  similar  cells,  in  which  the  differences  are 
entirely  and  permanently  accommodated  together,  the  further  result 
follows  that  the  hybrids,  like  any  other  stable  plant  species,  repro- 
duce themselves  truly  in  their  offspring.  The  reproductive  cells 
which  are  formed  in  their  seed  vessels  and  anthers  are  of  one  kind, 
and  agree  with  the  fundamental  compound  cell  [fertilised  ovum]. 

With  regard  to  those  hybrids  whose  progeny  is  variable  we  may 
perhaps  assume  that  between  the  differentiating  elements  of  the 
egg  and  pollen  cells  there  also  occurs  a  compromise,  in  so  far  that 
the  formation  of  a  cell  as  foundation  of  the  hybrid  becomes  possible; 
but,  nevertheless,  the  arrangement  between  the  conflicting  ele- 
ments is  only  temporary  and  does  not  endure  throughout  the  life  of 
the  hybrid  plant.  Since,  in  the  habit  of  the  plant,  no  changes  are 
perceptible  during  the  whole  period  of  vegetation,  we  must  further 
assume  that  it  is  only  possible  for  the  differentiating  elements  to 
liberate  themselves  from  the  enforced  union  when  the  fertilising 
cells  are  developed.     In  the  formation  of  these  cells  all  existing 

could  we  otherwise  explain  that  among  the  offspring  of  the  hybrids  both  original 
types  reappear  in  equal  numbers  and  with  all  their  peculiarities  ?  If  the  influence 
of  the  egg  cell  upon  the  pollen  cell  were  only  external,  if  it  fulfilled  the  role  of  a  nurse 
only,  then  the  result  of  each  artificial  fertilisation  could  be  no  other  than  that  the 
developed  hybrid  should  exactly  resemble  the  pollen  parent,  or  at  any  rate  do  so 
very  closely.  This  the  experiments  so  far  have  in  no  wise  confirmed.  An  evident 
proof  of  the  complete  union  of  the  contents  of  both  cells  is  afforded  by  the  experi- 
ence gained  on  all  sides  that  it  is  immaterial,  as  regards  the  form  of  the  hybrid, 
which  of  the  original  species  is  the  seed  parent  or  which  the  pollen  parent, 


APPENDIX  349 

elements  participate,  in  an  entirely  free  and  equal  arrangement,  by 
which  it  is  only  the  diflPerentiating  ones  which  nmtiially  sei)arate 
themselves.  In  this  way  the  production  would  be  rendered  i)o.ssible 
of  as  many  sorts  of  egg  and  pollen  cells  as  there  are  combinations 
possible  of  the  formative  elements. 

The  attribution  attempted  here  of  the  essential  difference  in  the 
development  of  hybrids  to  a  permanent  or  temporanj  union  of  the 
differing  cell  elements  can,  of  course,  only  claim  the  vahie  of  an 
hypothesis  for  which  the  lack  of  definite  data  offers  a  wide  scope. 
Some  justification  of  the  opinion  expressed  lies  in  the  evidence 
afforded  by  Pisum  that  the  behaviour  of  each  pair  of  difl'ercntiating 
characters  in  hybrid  union  is  independent  of  the  other  difi"erences 
between  the  two  original  plants,  and,  further,  that  the  hybrid  i)ro- 
duces  just  so  many  kinds  of  egg  and  pollen  cells  as  there  are  possible 
constant  combination  forms.  The  differentiating  characters  of 
two  plants  can  finally,  however,  only  depend  upon  differences  in 
the  composition  and  grouping  of  the  elements  which  exist  in 
the  foundation-cells  [fertilised  ova]  of  the  same  in  vital  inter- 
action.^ 

Even  the  validity  of  the  law  formulated  for  Pisum  requires  still 
to  be  confirmed,  and  a  repetition  of  the  more  important  experiments 
is  consequently  much  to  be  desired,  that,  for  instance,  relating  to 
the  composition  of  the  hybrid  fertilising  cells.  A  differential  [ele- 
ment] may  easily  escape  the  single  observer,^  which  although  at  the 
outset  may  appear  to  be  unimportant,  may  yet  accumulate  to  such 
an  extent  that  it  must  not  be  ignored  in  the  total  result.  Whether 
the  variable  hybrids  of  other  plant  species  observe  an  entire  agree- 
ment must  also  be  first  decided  experimentally.  In  the  meantime 
we  may  assume  that  in  material  points  an  essential  difference  can 
scarcely  occur,  since  the  unity  in  the  developmental  plan  of  organic 
life  is  beyond  question. 

In  conclusion,  the  experiments  carried  out  by  Kolreuter,  Gartner, 
and  others  with  respect  to  the  transformation  of  one  species  into 
another  by  artificial  fertilisation  merit  special  mention.  Particular 
importance  has  been  attached  to  these  ex])erimcnts  and  (liirtner 
reckons  them  among  '*  the  most  diflScult  of  all  in  hyl)ridisation." 

If  a  species  A  is  to  be  transformed  into  a  species  B,  both  must  be 
united  by  fertilisation  and  the  resulting  hybrids  then  be  fertilised 

1  "  Welche  in  den  Grundzellen  derselben  in  lehendiger  Wechselwirkung  stchen." 

2  *'  Dem  einzelnen  Beobachter  kann  leicht  ein  Dijjcrenziale  entgchcn." 


350  APPENDIX 

with  the  pollen  of  B;  then,  out  of  the  various  offspring  resulting, 
that  form  would  be  selected  which  stood  in  nearest  relation  to  B 
and  once  more  be  fertilised  with  B  pollen,  and  so  continuously  until 
finallj^  a  form  is  arrived  at  which  is  like  B  and  constant  in  its  prog- 
eny. By  this  process  the  species  A  would  change  into  the  species 
B.  Gartner  alone  has  effected  thirty  such  experiments  with  plants 
of  genera  Aquilegiay  Dianthus,  Geum,  Lavatera^  Lychnis,  Malva, 
Nicotiana,  and  Oenothera.  The  period  of  transformation  was  not 
alike  for  all  species.  While  with  some  a  triple  fertilisation  sufficed, 
with  others  this  had  to  be  repeated  five  or  six  times,  and  even  in  the 
same  species  fluctuations  were  observed  in  various  experiments. 
Gartner  ascribes  this  difference  to  the  circumstance  that  "  the 
specific  [typische]  power  by  which  a  species,  during  reproduction, 
effects  the  change  and  transformation  of  the  maternal  type  varies 
considerably  in  different  plants,  and  that,  consequently,  the  periods 
within  which  the  one  species  is  changed  into  the  other  must  also 
vary,  as  also  the  number  of  generations,  so  that  the  transformation 
in  some  species  is  perfected  in  more,  and  in  others  in  fewer  genera- 
tions." Further,  the  same  observer  remarks  ''  that  in  these  trans- 
formation experiments  a  good  deal  depends  upon  which  type  and 
which  individual  be  chosen  for  further  transformation." 

If  it  may  be  assumed  that  in  these  experiments  the  constitution 
of  the  forms  resulted  in  a  similar  way  to  that  of  Pisnm,  the  entire 
process  of  transformation  would  find  a  fairly  simple  explanation. 
The  hybrid  forms  as  many  kinds  of  egg  cells  as  there  are  constant 
i'  combinations  possible  of  the  characters  conjoined  therein,  and  one 
of  these  is  always  of  the  same  kind  as  that  of  the  fertilising  pollen 
cells.  Consequently  there  always  exists  the  possibility  with  all 
such  experiments  that  even  from  the  second  fertilisation  there  may 
result  a  constant  form  identical  with  that  of  the  pollen  parent. 
Whether  this  really  be  obtained  depends  in  each  separate  case  upon 
the  number  of  the  experimental  plants,  as  well  as  upon  the  number 
of  differentiating  characters  which  are  united  by  the  fertilisation. 
Let  us,  for  instance,  assume  that  the  plants  selected  for  experiment 
differed  in  three  characters,  and  the  species  ABC  is  to  be  trans- 
formed into  the  other  species  abc  by  repeated  fertilisation  with  the 
pollen  of  the  latter ;  the  hybrids  resulting  from  the  first  cross  form 
eight  different  kinds  of  egg  cells,  viz., 

ABC,  ABc,  AbC,  aBC,  Abc,  aBc,  abC,  abc. 


/, 


APPENDIX  351 

These  in  the  second  year  of  experiment  are  united  again  w  illi  the 
pollen  cells  abcy  and  we  obtain  the  series 

AaBbCc  +  AaBbc  +  AabCc  +  aBbCc  +  Aabc  +  ciBbc  -f  abCc  +  ahr. 

Since  the  form  abc  occurs  once  in  the  series  of  eight  terms,  it  is 
consequently  little  likely  that  it  would  be  missing  among  the  experi- 
mental plants,  even  were  these  raised  in  a  smaller  number,  and  the 
transformation  would  be  perfected  already  by  a  second  fertiHsa- 
tion.  If  by  chance  it  did  not  appear,  then  the  fertilisation  nuist  })e 
repeated  with  one  of  those  forms  nearest  akin,  Aabc,  ciBhr,  abCc. 
It  is  perceived  that  such  an  experiment  must  extend  the  farther  the 
smaller  the  number  of  experimental  plants  and  the  larger  the  number 
of  differentiating  characters  in  the  two  original  species;  and  that, 
furthermore,  in  the  same  species  there  can  easily  occur  a  delay  of 
one  or  even  of  two  generations  such  as  Gartner  observed.  The 
transformation  of  widely  divergent  species  could  generally  only  l)e 
completed  in  five  or  six  years  of  experiment,  since  the  numl)er  of 
different  egg  cells  which  are  formed  in  the  hybrid  increases,  as  the 
powers  of  two,  with  the  number  of  differentiating  characters. 

Gartner  found  by  repeated  experiments  that  the  respective 
period  of  transformation  varies  in  many  species,  so  that  frec^uently 
a  species  A  can  be  transformed  into  a  species  B  a  generation  sooner 
than  can  species  B  into  species  A.  He  deduces  therefrom  that 
Kolreuter's  opinion  can  hardly  be  maintained  that  "  the  two 
natures  in  hybrids  are  perfectly  in  equilibrium."  It  appears,  how- 
ever, that  Kolreuter  does  not  merit  this  criticism,  but  that  Giirtner 
rather  has  overlooked  a  material  point,  to  which  he  himself  else- 
where draws  attention,  viz.  that  '*  it  depends  which  individual  is 
chosen  for  further  transformation."  Experiments  which  in  this 
connection  were  carried  out  with  two  species  of  Pisum  demon- 
strated that  as  regards  the  choice  of  the  fittest  indiAiduals  for  the 
purpose  of  further  fertilisation  it  may  make  a  great  difi'erence  which 
of  two  species  is  transformed  into  the  other.  The  two  experi- 
mental plants  differed  in  five  characters,  while  at  the  same  time 
those  of  species  A  were  all  dominant  and  those  of  species  B  all 
recessive.  For  mutual  transformation  .1  was  fertilised  with  pollen 
of  B,  and  B  with  pollen  of  A,  and  this  was  repeated  with  both 

hybrids  the  following  year.    With  the  first  experiment  -  there  were 
eighty-seven  plants  available  in  the  third  year  of  experiment  for 


352  APPENDIX 

selection  of  the  individuals  for  further  crossing,  and  these  were  of 

A 

the  possible  thirty-two  forms;    with  the  second  experiment  — 

JB 

seventy -three  plants  resulted,  which  agreed  throughout  perfectly  in 
habit  with  the  pollen  parent;  in  their  internal  composition,  however, 
they  must  have  been  just  as  varied  as  the  forms  in  the  other  experi- 
ment. A  definite  selection  was  consequently  only  possible  with  the 
first  experiment;  with  the  second  the  selection  had  to  be  made  at 
random,  merely.  Of  the  latter  only  a  portion  of  the  flowers  were 
crossed  with  the  ^  pollen,  the  others  were  left  to  fertilise  themselves. 
Among  each  five  plants  which  were  selected  in  both  experiments 
for  fertilisation  there  agreed,  as  the  following  year's  culture  showed, 
with  the  pollen  parent : 


1st  Experiment 

2nd  Experiment 

2  plants 

in  all  characters 

3      " 

"4           " 

2  plants 

"    3            " 

2      " 

u    a              u 

1  plant 

"    1    character 

In  the  first  experiment,  therefore,  the  transformation  was  com- 
pleted; in  the  second,  which  was  not  continued  further,  two  or 
more  fertilisations  would  probably  have  been  required. 

Although  the  case  may  not  frequently  occur  in  which  the  dom- 
inant characters  belong  exclusively  to  one  or  the  other  of  the 
//  original  parent  plants,  it  will  always  make  a  difference  which  of  the 
two  possesses  the  majority  of  dominants.  If  the  pollen  parent  has 
the  majority,  then  the  selection  of  forms  for  further  crossing  will 
afford  a  less  degree  of  certainty  than  in  the  reverse  case,  which 
must  imply  a  delay  in  the  period  of  transformation,  provided  that 
the  experiment  is  only  considered  as  completed  when  a  form  is 
arrived  at  which  not  only  exactly  resembles  the  pollen  plant  in 
form,  but  also  remains  as  constant  in  its  progeny. 

Gartner,  by  the  results  of  these  transformation  experiments,  was 
led  to  oppose  the  opinion  of  those  naturalists  who  dispute  the 
stability  of  plant  species  and  believe  in  a  continuous  evolution  of 
vegetation.  He  perceives  ^  in  the  complete  transformation  of  one 
species  into  another  an  indubitable  proof  that  species  are  fixed 
within  limits  beyond  which  they  cannot  change.    Although  this 

^  ["  Es  sieht "  in  the  original  is  clearly  a  misprint  for  "  Er  sieht."] 


APPENDIX  '35S 

opinion  cannot  be  unconditionally  accepted,  we  find  on  the  other 
hand  in  Gartner's  experiments  a  noteworthy  confirmation  of  that 
supposition  regarding  variability  of  cultivated  plants  which  has 
already  been  expressed. 

Among  the  experimental  species  there  were  cultivated  plants, 
such  as  Aquilegia  atropurpurea  and  canadensis,  Dianthus  caryophyl- 
lus,  chinensisy  and  japonicus,  Nicotiana  rustica  and  paniciilaia,  and 
hybrids  between  these  species  lost  none  of  their  stability  after  four 
or  five  generations. 


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Zeleny,  C,  and  E.  W.  ;\L\ttoon,  1915.   The  effect  of  selection  up<jn  the 

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ZiNN,  J.,  1919.   On  variation  in  Tartary  buckwheat.   Genetics,  4. 


INDEX 


Acclimatization,  31. 

Acquired  characters,  gO,  22,  28,  45. 

Agouti,  115.  124,  188. 

Albino,  24,  88,  122a. 

Alcohol,  effects  of,  on  germ-cells,  45. 

Allelomorph,  101 . 

Allelomorphs,  multiple,  186. 

Allen,  139. 

Andalusian  fowl,  109. 

Angora,  91,  127. 

Ants,  53. 

Apotettix,  176. 

Apple  graft-hybrid,  23. 

Arcella,  210. 

Ascaris,  51. 

Atavism,  113. 

Average  deviation,  64 

Barnacle,  light  reactions  of,  43. 

Barrows,  140. 

Basset  hounds,  246. 

Bateson,  55,  92,  109,  120,  136,  156,  173. 

Baur,  23,  113,  211. 

Beagle  J  voyage  of ,  9,  14. 

Beans,  selection  for  size  in,  215. 

Bees,  53. 

Bert,  32. 

Binet  test,  286. 

Biometry,  55,  57,  62. 

Birthrate,  differential,  295. 

Bison,  133,  236. 

Black  coat,  124. 

Blending  inheritance,  192,  246. 

Bond,  275. 

Bonhote,  124. 

Bos,  230. 

Bounty,  mutineers  of  the,  207. 

Bo veri,  51,249. 

Bridges,  219,  249. 

British  aristocracy  and  eugenics,  297. 

Brown  coat,  124. 

Brown-Sequard,  30. 


Buffon,  18. 
Buttercup,  63,  73. 
Butterfly,  52. 

Castration,  effects  of,  2.')1. 
Cats,  short-tailed,  2H,  143 

unit-characters  of,  142. 
Cattell,  29.),  298, 
Cattle,  polled,  91,  133. 

short-horn,  110. 

unit-characters  of,  130. 

white,  of  English  parks,  131, 

wild,  130. 
Cave  animals,  40. 
Cavia  Cutleri,  243. 
Cavia  rufescens,  256. 
Checkerboard  method,  105,  116. 
Cheledonium,  152. 
Child,  52. 
Chimera,  23. 
Chromosomes,  49,  51. 

changes  in,  205. 
Chromosome  map,  168. 
Ciona,  29,  229. 
Coefficient  of  correlation,  65. 
of  inbreeding,  238. 
of  variation,  65. 
Coleus,  149,  211. 
Collins,  262. 
Colorblindness,  162. 
inheritance,  88. 
Colors  of  flowers,  148. 

of  fruits,  151. 
Columba,  114. 
Congenital  disease,  29. 
Conn,  11. 
Correlation,  65. 
Correns,  82. 
Crepidula,  253. 
Crossing-over,  168. 
Cuenot,  111. 
Curve  of  error,  60,  62*,  72. 


391 


392 


INDEX 


Daphnia,  32,  213,  255. 
Darwin,  Charles,  7,  48,  55,  83,  113,  228. 
Darwin,  Erasmus,  9. 
Davenport,  271,  275,  277,  284. 
Davis,  80. 
Delage,  34. 

Determiners,  48,  49,  50,  53,  54. 
Detlefsen,  179. 
Diffliigia,  209. 
Dihybrid  ratio,  107,  116. 
Dilute  pigmentation,  127. 
Disease,  inheritance  of,  29. 
Dogs,  unit-characters  of,  138. 
Domestication,  changes  under,  14. 
Dominant,  88. 
Drinkwater,  190. 

Drosophila,  156,  176,  180,  188,  219,  224, 
230,  249. 

eye  colors  of,  219. 
Ducks,  size  inheritance  in,  197. 

East,  193,  199,  210,  242,  244. 

Egg-cell,  98. 

Egypt,  royal  family  of,  227. 

Elementary  species,  74. 

Embryology,  12. 

Emerson,  199,  211,  224. 

Environment,  direct  effect  of,  20. 

Epilepsy,  30,  284. 

Eugenics  defined,  3. 

Eugenics  Laboratory,  271. 

Eugenics  Record  Office,  271. 

Evening  primrose,  49,  75,  220. 

Evolution  defined,  4,  7. 

history  of,  repeated  in  devel- 
opment, 13. 

Extension  factor,  188. 

Factor,  modifying,  189. 

multiple,  192. 
Farabee,  190. 
Feeble-mindedness,  285. 
Ferroniere,  32. 
Filial  generation,  100. 
Fischer,  34,  269. 
Fixation  of  new  varieties,  95. 
Flower  colors,  148. 

forms,  150. 


Fluctuations,  71,  78. 
Free  martin,  252. 
Fruit  colors,  151. 

Galapagos  islands,  14. 

Gall  insects,  52. 

Gallus  bankiva,  145. 

Galton,  3,  9,  26,  56,  71,  246,,  295. 

Galton's  law  of  ancestral  heredity,  239, 246. 

Gamete,  98. 

Gates,  50. 

Gene,  99. 

Genes,  linear  arrangement  of,  168. 
nature  of,  177. 

Genetic  changes,  205. 

in    bisexual    reproduc- 
tion, 219. 

in     parthenogenesis, 
212. 
in  self-fertilization,  215. 

Genetics  defined,  3. 

Genotype,  102. 

Geographical  distribution,  13. 

Geological  succession,  13. 

Germ-cells,  23,  47. 

Germinal  selection,  54. 

Gipsy  moth,  33. 

Goddard,  285. 

Goodale,  251. 

Goss,  86. 

Gould,  253. 

Gradation  of  organisms,  12. 

Graft  hybrid,  23. 

Greek  philosophers,  8. 

Gross,  202. 

Guinea-pig,  24,  30,  88,  91, 114, 122a,  187. 

Gynandromorph,  249. 

Haemophilia,  162. 

Hair  form,  inheritance  of,  275. 

Haldane,  170,  173. 

Harrison,  23. 

Hegner,  51,  209. 

Height  of  Harvard  students,  57,  61. 

Heron,  281,  283. 

Hermaphroditism,  261. 

Hertwig,  45. 

Heterosis,  242. 


INDEX 


393 


Heterozygote,  98. 
Heterozygous,  90. 
Homozygote,  98. 
Homozygous,  90. 
Hormones,  252. 
Horse,  ancestor  of,  13. 

Clydesdale,  136. 

Prevalski's,  134. 

Shire,  136. 

unit-characters  of,  134. 
Hoshino,  200. 

Human  crosses,  English-Polynesian,  268. 
Boer-Hottentot,  269. 
Jew-Anglo  Saxon,  274. 
Negro-white,  271. 
Huxley,  12. 
Hybrid,  79,  83. 
Hydatina,  254. 

Illiteracy,  290. 

Immigration,  290. 

Inbreeding,  222,  223,  224,  227,  230. 

Insanity,  280. 

Instinct,  41. 

Intense  pigmentation,  127. 

Jennings,  209,  240. 
Johannsen,  94,  208,  247. 
Jones,  244. 
Jungle  fowl,  145. 

Kammerer,  29,  37. 

Kellogg,  33. 

King,  129,  222,  231,  238,  256. 

Knight,  86. 

Kolreuter,  83. 

Lamarck,  17,  18,  47. 
Leaves,  forms  of,  152. 
Light  effects,  38. 
Lillie,  252. 
Linkage,  120,  167,  180. 

chromosome  theory  of,  168. 

measurement  of,  172. 
Little,  111,  125,  139,  179. 
Lizard,  37. 
Lock,  12,  83. 
Loeb,  43,  256. 


Lychnis,  262. 
Lyell,  8. 

MacDowell,  224. 

Macroganiele,  253. 

Maize,  151,  174,  ]f)l,  211,  223,  228. 

Malthus,  15. 

Marriage  of  noar-of-kin,  227. 

McClung,  200. 

Mean,  64. 

Memlcl,  82,  190,  248. 

Mendel's  law,  55,  82,  88. 

Mendelian  expectations,  104. 

ratios,  nioflified,  109,  110,  119. 
Mental  ability,  inheritance  of,  279. 
Miastor,  51. 
Microgamete,  254. 
Microtus,  124. 
Miller,  139. 
Mirabilis,  86,  110,206. 
Mode,  59. 
Mongrel,  83. 

Morgan,  52, 120,  249,  251. 
Morphology,  13. 
Moth,  brown-tail,  43. 
Mulatto,  277. 
Mule- footed    swine,  138. 
Multiple  allelomorphs,  187,  219. 
Multiple  factor  hypothesis,  192. 
Mus alexandrinus,  125,  . 

rattus,  124. 
Mutants  of  Oenothera,  76* 
Mutation,  14,  71, 182, 185, 189. 

varieties  of,  208. 
Mutilations,  28. 


Nabours,  176. 

Natural  selection,  11,  17,  56. 

Naudin  87. 

Negro,  white-spotted,  277. 

Nicotiana,  85,  229,  236,  242. 

Nilsson-Ehle,  193, 


Oenothera,  49,  75,  221. 
Origin  of  Specieti,  11,  17. 
Osborn,8,  19,266. 

Ovarian  transi)hintalion  in  fowls,  251. 
in  guinea-pigs,  24. 


394 


INDEX 


Ovarian  transplantation  in  rats,  251. 
Overproduction,  11. 
Oxford  "class  lists,"  279. 


Pangenesis,  24,  26,  48. 
Parallel  induction,  35,  54. 
Paramecium,  selection  for  size  in,  209. 
Parental  generation,  100. 
Parthenogenesis,  212. 

in  distant  crosses,  236. 

in  frog,  256. 

in  honey-bee,  257. 

in  orange,  257. 

in  sea-urchin,  237. 

male,     in     strawberry, 

237. 

in  teosinte,  237. 
Pasteur,  29. 
Payne,  225. 
Pearl,  238. 
Pearson,57,  68,  279. 
Pearson,  57,  68,  279. 
Peas,  dwarf  and  tall,  152. 

flowering  time  of,  200. 
linkage  in,  174. 
Pebriue,  29. 
Pelargonium,  211. 
Peppers,  size  inheritance  in,  202. 
Peromysciis,  123,  125. 
Phenotype,  94,  102. 
Phillips,  24,  125,  140,  179,  197. 
Pictet,  33. 

Pink-eyed  rodents,  126. 
Pitcairn  Island,  2G8. 
Plants,  unit-characters  of,  149. 
Plato's  Eepublic,  292. 
Plums,  self-sterility  in,  229. 
Polymorphism,  52. 

Polynesians  and  venereal  disease,  305. 
Potato  beetle,  35. 
Poultry,  unit-characters  of,  145. 
Pressure  effects,  38. 
Prevalski's  horse,  134. 
Prunula,  150,  173. 
Probable  error,  68. 
Proteus,  40. 
Prune,  new  variety  of,  210. 


Punnett,  104,  109,  120,   125,  145,  173, 

198. 
Pure  line  hypothesis,  215. 

Rabbit,  26,  67,  191. 

Dutch-marked,  125,  224. 

ear-length  of,  194. 

Enghsh,  125,  183. 

gray  coat  of,  205. 

Himalayan,  138. 

size  inheritance  in,  191. 
Race  mixture  and  venereal  disease,  306. 
Rat,  117,  176,  222,  230. 

hooded,  184,  223. 
Regeneration,  51. 
Regression,  247. 
Reversion,  113. 
Riddle,  255. 
Roan  cattle,  110. 
Rock  pigeon,  114. 
Rodents,  unit-characters  of,  122. 
Romanes,  31. 
Rosanoff  and  Orr,  281. 
Rough  coat  of  guinea-pigs,  128. 
Ruby  eye  of  rats,  129. 

Saint-Hilaire,  18. 

Salaman,  274. 

Salamander,  37. 

Salinity,  altered,  32. 

School  rank  and  success  in  later  life, 

279. 
Sebright  bantam,  251. 
Segregation,  91. 
Selection,  71,  184,  189,  222,  223. 

in  Aphis  avenae,  214. 

in  beans,  215. 

in  Daphnia,  213. 

in  hooded  rats,  184. 

in  Paramecium,  209. 

in  parthenogenesis,  213. 

in  pure  lines,  215. 

in  Simocephalus,  214. 
Self-fertilization,  75,  223,  228. 
Self-pollination,  228. 
Self-sterility,  229. 
Semon,  29,  41. 
Sex-chromosome,  157. 


I 


I 


INDEX 


31), 


Sex-determination  of  bee,  257. 

of  Crustacea  and  roti- 
fers, 258. 
of  squash-bug,  259. 

Sex-linked  inheritance,  159. 

Drosophila  t^TJe, 

1G4. 

poultry  type,  165 

Sex-intergradcs,  213. 

Sex-mosaic,  24'9.  , 

Sex  ratio  in  rats,  222. 

Shamel,  210. 

Sheep,  docking  of,  28. 

unit-characters  of,  138. 

Shepherd's-purse,  193. 

Shull,  193,  228,  262. 

Silkworm,  unit-characters  of,  154. 
linkage  in,  176. 

Simocephalus,  213. 

Skin-color,  inheritance  of,  277. 

Snapdragon,  113,  174. 

Soma,  23. 

Spartan  eugenics,  292. 

Spencer,  Herbert,  25. 

Sperm,  98. 

Standard  deviation,  64. 

Steinach,  251. 

Sterility  in  crosses,  233. 

Stockard,  45. 

Stout,  211. 

Struggle  for  existence,  11. 

Sugar  beet,  71. 

Sumner,  37. 

Survival  of  the  fittest,  11. 

Sweet  peas,  118,  120,  149,  173,  218. 

Swine,  unit-characters  of,  137. 

Syphilis,  30. 

Temperature  effects,  34. 
Texas  fever,  29,  207. 
Tomato,  linkage  in,  174. 
Tower,  35. 
Toyama,  154. 
Transfusion  of  blood,  26. 
Transplantation  of  ovaries,  24,  28,  251. 
Treasury  of  Human  Inheritance,  271 


Tri-colorcoat,  12G. 
Tri-hybrid  ratio,  107,  119. 
Tsrhcrni.'ik,  82. 
Tuberculosis,  30. 
Tubifex,  32. 

Unit-character,  91,  99,  182. 
Unit-factor,  99. 
Use  and  disuse,  20. 

Variation,  11,  19,  21,  53,  55,  71. 
Variation  of  Animals  and  Plants  under 

Domestication,  14,  17,  19,  24,  56. 
Variation,  coefficient  of,  05. 

continuous  or.  fliscontinuous, 
56. 
Variation  curve,  59. 
Vasectomy,  291. 
Vries,  H.  de,  56,  71,  82. 

Wallace,  10. 

Wedgewood,  9. 

Weight  of  Harvard  students,  57,  61,  63. 

Weismann,  21,  29,  34,  41,  47,  231. 

Wentworth,  135. 

Wheat,  crosses  of,  192. 

Whetham,  297. 

White  spotting,  125,  188. 

Whiting,  129. 

Whitney,  254. 

Wilson,  259. 

Winkler,  23. 

Wislar  Institute,  231. 

Wolves,  coat  color  of,  140. 

Wright,  128,  177. 

X-chromosome,  157,  159,  260. 

Y-chromosome,  260. 
Yellow  coat,  124. 
Yellow  mice,  111. 
Yellow  spotting,  126. 
Yule,  63. 

Zeleny,  225. 
Zygote,  98. 


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